If i find a pending edit to a post while browsing, but have already reviewed 20 edits that day from the queue, i'm unable to review that edit.
I'm pretty sure that the reason for limiting to 20 reviews is so that people don't get burned out of reviewing. That's a good idea.
I think though, that if you come across an edit while browsing, approving or rejecting it shouldn't burn you out.
Doing this wouldn't even have to count as a review, so noöne could review 21 edits in a day, and it would just be altruistic. Or else, it could count as a review.