Brevet rank is real rank, but without an army position in that pay. Scott was really a Lt Gen, but since there were no Lt Gens on the US Army orgchart he only filled the Maj Gen slot (the only one). In 1861 with the appointment of several more Maj Gens the rank of Maj Gen commanding was created.
I'm interested as to whether McClellan ever wore the three stars he was entitled too. Given his humble character one suspects not, hence my search for an image of it.