Incorrectly experience point gain

What's happened:
The experience point gained may different that I really get.

What's I'm expecting:
The experience point gained is what I really get.

How to reproduce:

1. experience set @s 100 levels
2. experience add @s 6 points
3. experience query @s points
   -> 5 exp points

1. experience set @s 100 levels
2. experience add @s 12 points
3. experience query @s points
   -> 11 exp points

1. experience set @s 100 levels
2. experience add @s 24 points
3. experience query @s points
   -> 23 exp points

1. experience set @s 100 levels
2. experience add @s 48 points
3. experience query @s points
   -> 47 exp points

1. experience set @s 100 levels
2. experience add @s 7 points
3. experience query @s points
   -> 7 exp points