I'm not sure if this is a bug or the way it was intended so I guess I'll just report it and let Ryme say if this was how it was meant to be.
The XP gain from clockwork spider seems to be level dependent. However, I'm not sure the ranges in practice are what was meant. Here's what I saw.
I think with more experiencing it might reveal to be 8-20 for everything level 7+, but that's what I got so far in 3 transmogs.
I was careful not to use any familiar efficiency modifier.