OK, it looks like I eventually found my own answer: it's because the cells have a "full well" capacity limit in practice...
But you'd never do that, you'd just record the low 4 bits too, and then just discard them in software if you didn't like them.
-
-
So there has to be more to it than that.
Thanks. Twitter will use this to make your timeline better. UndoUndo
-
Loading seems to be taking a while.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.