I measured the top tube from center of the head bracket to the center of the seat tube--I get 21" here--that's what's been so confusing to me considering all the other frame sizes. As has been pointed out I need to get a metric tape measure.
I looked at at Trek 470 with a geometry almost exactly like mine--the only difference was a 31" stand over, and this bike had a 56 cm frame sticker. The owner could have mismeasured I guess.

Thanks for the reply.