Actually, I think that when you do a hot full database backup through RMAN, rman internally would do an alter database backup control file option. On the other hand when you do an OS level hot backup, you would temp be taking the controlfile off. This might cause corruption, as control file consistently keeps track to the SCN#, which gets written on to it. By temp inturuption, you might get skip some of the SCN#s that needs to be recoded, in a sequence. This SCN# is used at the time of recovery to see what order the instance had to call the archive log file during recovery.

Hope I'm close.
Sam