when you use execute to summon an Entity it summons the Entity realitve to the blockgrid not the Entity itself

What I expected to happen was...:
The entity to be summoned where the player was

What actually happened was...:
The entity was summoned on the blockgrid instead

Steps to Reproduce:
1. stand on the edge of a block
2. run this command: /execute @p ~ ~ ~ summon Pig ~ ~ ~
3. Watch the pig be summoned realitve to the block and not the player