I'm guessing something about your bike did not meet the specifications on that page somehow.
it is more likely than not much easier to get your build conforming to that definition than it would be to get it registered as a motorcycle.
I just re-read the page, even mopeds need a plate. says it's five dollars to register at you local county tax collection office.
if anyone from Florida knows better feel free to chime in here.
lame but by the sounds of things still legal to use if you jump the right hoops.
Stay in there. Paperwork ain't that hard.