Actually the reason is cause it is a toshiba....Many of the toshiba dvd-rom drives were locked to a 2x dvd-rip through the firmware....12x is a read speed and has nothing to do with dvd-rip speeds....
These rip speeds can fluctuate greatly from programs, media type, format (ie dual layer vs single layer), etc.....BUt for the most part if you seem to max out at 2.0x then it is what I described above....
That is why hands down the best dvd-rom on the market still remains to be the lite-on 16x dvd-rom...I hits speeds of 12x plus sometimes....