Potions don't give specified time when thrown at certain positions

I just got a potion and threw it, supposed to last 0.33 sec, it started at 0.29 sec. (Regen)
I discovered that when throwing it from above it will give the exact time; as throwing it from below gives about 5% less time than it should be.
Steps to Reproduce:
1. Get a splash potion
2.Throw from below and immidietly open your inventory
3.Throw from above and immidietly open your inventory again
4.Notice the changes