Florida is a bit of a mystery where monarchs are concerned - there does appear to be a resident population throughout the winter in some areas, but just as you have observed there are many reports of dwindling numbers of monarchs over the winter that are replenished in the late fall. Through our tagging program, we do know that some monarchs that begin the migration further north in the U.S. and Canada end up in Florida rather than Mexico, presumably becoming a part of a resident population. Check out our Tag Recovery Database
(select Florida as the recovery state and leave everything else blank) to see these records - you can specify a sort by decending distance for a more sophisticated search that makes this even easier to see.
Even the monarch experts don't completely grok monarchs - hmmm...is it proper to speak of degrees