You may have reverted to older firmware in the past but that's because either you have an older phone (3G and below have no signature checking), did so before A

You may have reverted to older firmware in the past but that's because either you have an older phone (3G and below have no signature checking), did so before Apple started signing firmwares (I believe it started somewhere in 3.x), or you're caching the signatures using TinyUmbrella, Saurik's caching server, SHSHit, or something similar. Apple DOES sign firmwares now and generally refuses to sign older versions soon (couple of hours, maybe a day or two) after a fresh release.