TNT in Minecart with TNT swells in offset direction

The Bug:

Pay close attention to the block inside as it gets bigger, it appears to get bigger in a corner direction instead of centered on the block.

Steps to Reproduce:

1. Switch into creative mode, place down an activator rail and ensure that it's powered.
2. Place a TNT minecart on the activator rail and watch how the TNT swells from above.
3. Take note as to whether or not TNT in TNT minecarts swells in offset directions.

Observed Behavior:

TNT in TNT minecarts swells in offset directions.

Expected Behavior:

TNT in TNT minecarts would not swell in offset directions. Instead, it should swell according to the center of the block and not the corner.

Code Analysis:

Code analysis by Ismael Rosillo can be found in this comment.