Yes - the hardware in the two drives is different. Hence, the firmware to control the chipset and the PUH is compiled and written differently.
There is conjecture and hearsay about the difference in both drives write quality. The general consensus was, from memory, that the x51S series drives were simply cheaper to produce. Others say the x51S series perform better. Subjective I guess.
The newer series again from LiteOn, the SOHW-xxxx line are different hardware again, using the Sony PUH. Oddly enough, still uses the MT1818E chipset however!