I wanted to summon a villager with custom trades and without AI so I took a command block, placed it down and started typing the command. Notice I used relative coordinates:

/summon Villager ~ ~1 ~ {NoGravity:1,NoAI:1}

What I expected to happen was:
when I pressed the button on the command block, it would summon the villager standing on the command block.
This also happens when using /execute .

What actually happened was:
the villager summoned but not 1 block above the command block, but 1,5 block above.

Tried with other mobs and also items, it did the same. When I used ~ ~0.5 ~ it did summon exactly on the command block. Also, this doesn't happen with absolute coordinates, only with relative.

How to reproduce:
1. Give yourself a command block
2. Go into the command block and put in this: /summon <your entity> <your relative coordinates>
3. Attach a button to the command block and press it
4. You'll see it is 0.5 block higher than where you actually wanted your entity to be.