Event AEE not so A?

stuff

How do you have 256 in the first place? It is not the end of the byte; the byte ends at 255. For indisputable clarity, let's go back to first grade:
First number: 0
Second number: 1
Third number: 2
2^8th number: 2^8 - 1
Perhaps my lack of binary math experience is taking its toll on me, so please enlighten me in case I'm missing something (except the fact that 256 comes right after 255, I'm well aware of that).

EDIT: Allow me to make clear distinctions here, when people say they failed 100% scrolls (presumably the blue ones), did you see the failure animation or did your total bonus stats end up one below the expected value? Because the latter does not imply the earlier and I surely have seen it happen myself.