maybe there are different manufacturers?
All I can say is this: My ears don't lie and I had to listen to 3 different RNS 510 units before I settled on mine.
At last I have been vindicated. To put it mildly, the RNS 510 that I have in my car sounds crap but that doesn't mean they all do, just mine in particular. The RCD 310 looks a bit unattractive but sounds brilliant.The RNS 510 in my car is unusable for listening purposes unless fitted with a sub-woofer and separate amp. The RCD 510 is no better IMHOIt is for this reason I have fitted an underseat sub-woofer and will be attaching a power amp tomorrow to the RNS 510.