Energy charge bars?

I've noticed lately that the energy charge bar seems to dance around a bit. I know that sometimes when one takes a hit, one gains some energy. But lately, I've also see it drop. I'm tapping away, trying to build up for the charge attack, and suddenly it drops some.

Is there something about maybe the type of damage affecting the energy level? Or is it just a software issue or lag? I doubt that it's me dodging and the 'expected' damage not happening, I seldom do that, especially when I should win without it.

Any insight appreciated.