Ok, so apparently, voller can be used in ALL cases! If you want to go full language geek, there is an entire research paper on it here http://corpling.uis.georgetown.edu/amir/pdf/case_for_caseless.pdf
Why will Duolingo sometimes allow for multiple mistakes but others it wint allow a single mistake? My stinking autocorrect put ist in place of is and counted this wrong. Obviously ist is German. Why the silly punishment when I've made more than 3 spelling errors in other sentences but Duolingo merely says "you have some typos"?
I have the feeling that spelling mistakes need to be manually added by the moderators on a per sentence so one sentence may have more "tolerance" than another.
Or maybe they're just trying to toughen things up by allowing fewer because we're getting further up (or down, rather) the tree. I get the feeling it's the former, though.