Don't kill the messenger, lol.
I have the same questions myself. But a random guess would be cost. Cheap tires for OEM
Quick Googling show's that there were different letter designations and the difference was like RockeryGuy said, # of compounds. BT016-E BT016-M I forget which is OEM and which is aftermarket, but there are dozens of threads on GSXR forums about this very thing.