Sandforce drives were 120GB because they used that extra 8GB to distribute writes, so that the flash would last longer. Marvel didnt, so it had that extra capacity.
No reason why Samsung couldn't do that on their own controller. The 840 uses TLC flash which degrades quicker, so sacrificing that 8GB for write distribution on that drive and not the 840 Pro (uses MLC flash, which is more resistant) makes sense.