Android: I can't seem to get my custom toolbar's back arrow to act like the device's physical back arrow

I have the back arrow implimented and working but instead of just finishing the activityB and going back to the previous view the user was on in activityA. It instead closes activityB and reloads activityA which means you have to go back through a few menus to get back to where you were.

The physical back arrow does exactly as I want it to in that it just closes the activtyB and puts the user back to where they were within activtyA originally before they moved to activtyB.

This is my current code for the toolbar back arrow but maybe I'm just not fully grasping how the physical back arrow works. I appreciate any and all advice!

toolbar = (Toolbar) findViewById(R.id.tool_bar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);