There's a couple of reasons DCC runs slower than DC.
Firstly, the DCC signal is modulated AC so it can never deliver the same voltage as a similarly rated DC controller, it loses a bit more as it's run through the rectifiers in the decoder. Also, the decoder itself needs a certain amount of power just to keep it "ticking over". Typically when you run a DCC equipped loco on a DC layout you have to up the start voltage to get it moving. (I tried it with a Zimo sound decoder on DC and the loco drew 7 volts before the sound came on.)
Can't answer your second question, maybe it's just a manufacturer thing....