At least these are different GT 555M's
•144 cores 709MHz (GF106), 128Bit GDDR5
•144 cores 590MHz (GF106), 192Bit DDR3
•144 cores 590MHz (GF106), 128Bit DDR3
•96 cores 753MHz (GF108), 128Bit GDDR5
•144 cores 525 MHz (GF116), 128 Bit DDR3
Wether performance of card X's retail and OEMs is in the same ballpark is hard to say, since OEM versions benchmarks are hard to come by, but just from the latest gen at least GTX660 and GTX660 OEM use different GPUs and different configurations
OK, I admit the difference between the first and the last one has to be pretty massive. I guess NVIDIA gets away with this because enthusiasts don't really pay attention to these cards.