Slimes are not positioned correctly. They are offset slightly north-west-up. I can't think of any other way to explain this, so I made a short video here: https://youtu.be/8DUTGyhvL8w

How to reproduce -
Place down a block
Summon a slime within the block, using the command

   /setblock ~ ~ ~2 stone
   /summon Slime ~ ~ ~2 {NoAI:1,Invulnerable:1,Size:1}

Look at the slime. It's slightly sticking out of the block, but not by the same
amount from each side.