If you dig deep enough, you'd probably find they all use the same parts or similar parts (for the same specifications). All these RAM chips come only from a few semiconductor factories. It's not like Brand-X has their own chip foundry.
The rest is just marketing and buyer perception... i.e. what is the flavor of the month that PC builders are in love with right now? Who has the best marketing/PR at the moment? Give it a few more months, and when too many people are buying the same thing, someone will say hey check out this new memory thingamajig model NANO-CRYSTAL-JAWS-XTREME... it's flat black, very bad ass looking... it's better than the others.