Each manufacturer builds it differently so different make of RAM can offer you better quality.
Corsair and OCZ are the top RAM manufacturers. OCZ's newest RAM the Platinum Series Rev2, have outperformed Corsairs XMS memory in damn near every benchmark and also has way better overclockability.
Samsung does make good RAM sticks, if you look at your stick whether it be corsair, kingston, or whatever you want, generally if you look at one of the RAM chips close up you'll see it says its made by Samsung most of the time
So they not only make their own RAM sticks but they also make quality RAM CHIPS that other manufacturers put into their RAM sticks.