The hitbox of newly spawned slimes is offset for a short amount of time. This is easiest to see by summoning slimes with NoAI, but it also affects slimes with AI. Slimes spawned with spawn eggs are also affected. I assume this occurs with naturally spawned slimes too.

To reproduce

1. Enable hitbox debug (F3 + B)

2. /summon Slime ~ ~ ~ {NoAI:1b}

3. Look at the hitbox of the slime