The XP orb collision hitbox is off-centre on the X-axis by around 0.125 (not exactly!) blocks.

Steps to reproduce:

1. Find a source of XP orbs, such as a command block.
2. Use water to push the XP orbs against walls in each of the 4 directions.
3. Stand directly above the centre of the setup and high enough that the player doesn't collect or attract the XP orbs.

Expected behaviour

The XP orbs stop at equal distances from the wall in all 4 sides.

Observed behaviour

The XP orbs will appear to clip into the wall on the +X side, and stop further away from the wall on the -X side. The expected behaviour is observed on the Z axis. In the picture below, the red concrete marks the +X axis, and the blue concrete is the +Z axis.