How does experience work for skills in world of tanks?

How does experience work for skills in world of tanks?

I've got a tank that has just reached 100% crew experience level and, all
excited, I set some skills to advance.
My Commander had a skill "Signal Boosting" and I received 500 experience
points for a match; all my experience is being sent into "accelerate crew
training" and it says that I only need 72 xp to "Upgrade Skill" (estimated
one battle)
But after the 500 xp my skills only went up to 12%, what's happening here?
Even with 500 xp divided amoungst the 4 crew that would seem to be >72 xp
needed!