Why the checkbox state isn't consistent?

package com.example.aakarshrestha.topiapp;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    ListView mListView;
    ArrayList<UserProfile> mUserNameArrayList;
    CustomAdapter mAdapter;
    Button mButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mUserNameArrayList = getAllUserNames(); //create a function to get data in list

        mListView = (ListView) findViewById(R.id.listView);
        mAdapter = new CustomAdapter(this, mUserNameArrayList);
        mListView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
        mListView.setAdapter(mAdapter);

        mButton = (Button) findViewById(R.id.button);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                refresh();
            }
        });

    }

    private ArrayList<UserProfile> getAllUserNames() {

        ArrayList<UserProfile> nameArrayList = new ArrayList<>();

        UserProfile userName = new UserProfile();
        userName.setUsername("Apple");
        nameArrayList.add(userName);

        UserProfile userName1 = new UserProfile();
        userName1.setUsername("Microsoft");
        nameArrayList.add(userName1);

        UserProfile userName2 = new UserProfile();
        userName2.setUsername("Linux");
        nameArrayList.add(userName2);

        UserProfile userName3 = new UserProfile();
        userName3.setUsername("HP");
        nameArrayList.add(userName3);

        UserProfile userName4 = new UserProfile();
        userName4.setUsername("Dell");
        nameArrayList.add(userName4);


        return nameArrayList;
    }

    private void refresh() {
        mListView.setAdapter(mAdapter);
    }
}

package com.example.aakarshrestha.topiapp;
import android.content.Context;
import android.content.SharedPreferences;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.CheckBox;

import java.util.ArrayList;

public class CustomAdapter extends BaseAdapter{

    Context mContext;
    private  static ArrayList<UserProfile> mUserNameArrayList = new ArrayList<>();
    private LayoutInflater mInflater;
    ViewHolder mHolder;
    SharedPreferences mSharedPreferences;

    public CustomAdapter(Context context, ArrayList<UserProfile> userNameArrayList) {
        mContext = context;
        mUserNameArrayList = userNameArrayList;
        mInflater = LayoutInflater.from(mContext);

        mSharedPreferences = context.getSharedPreferences(MainActivity.class.getSimpleName(),
                Context.MODE_PRIVATE);
    }

    @Override
    public int getCount() {
        return mUserNameArrayList.size();
    }

    @Override
    public Object getItem(int position) {
        return mUserNameArrayList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {

        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.listview_item, null);
            mHolder = new ViewHolder();
            mHolder.mCheckBox = (CheckBox) convertView.findViewById(R.id.checkBox);
            convertView.setTag(mHolder);

        } else {
            mHolder = (ViewHolder) convertView.getTag();
        }

        mHolder.mCheckBox.setChecked(mSharedPreferences.getBoolean(mUserNameArrayList.get(position).getUsername(), false));

        mHolder.mCheckBox.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                SharedPreferences.Editor editor = mSharedPreferences.edit();

                if (mHolder.mCheckBox.isChecked()) {
                    editor.putBoolean(mUserNameArrayList.get(position).getUsername(), true);
                    editor.apply();
                } else {
                    editor.putBoolean(mUserNameArrayList.get(position).getUsername(), false);
                    editor.apply();
                }
            }
        });


        mHolder.mCheckBox.setText(mUserNameArrayList.get(position).getUsername());
        return convertView;
    }

    static class ViewHolder {
        CheckBox mCheckBox;
    }
}

package com.example.aakarshrestha.topiapp;

public class UserProfile {

    private String username;
    private boolean isSelected;


    public UserProfile() {
    }

    public UserProfile(String username, boolean isSelected) {
        this.username = username;
        this.isSelected = isSelected;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public boolean isSelected() {
        return isSelected;
    }

    public void setIsSelected(boolean isSelected) {
        this.isSelected = isSelected;
    }

    @Override
    public String toString() {
        return username;
    }
}