can someone explain this ?

at level 888 i get 948 XP for hitting the world boss with 20 stamina.
at level 892 i get 774 XP for hitting the world boss with 20 stamina.

how is this possible ?

 

I think it's random and depends on your health just like a regular boss would.

I never get the same amount of experience from an attack against bosses.

 

right. forgot about that still, it's a big difference, almost 200 XP.