Could be a lot of things.. if it's an active adapter then the video signal is processed from one format to another, like displayport to hdmi, etc. They don't all use the same chip. And while I don't have a KVM I am using an adapter.. my PC has DP and HDMI outputs, but I'm using an DP to HDMI adapter because the HDMI output on the PC doesn't look as nice. It was the same thing with the Dock I have for my deck.. the HDMI output from the dock's converter wasn't as good as a DP to DVI adapter I got.. so, the switch probably just has a chip or something that doesn't suck.
Another could be the video timings.. there's the timing that is stuffed into the monitor EDID, and there are other timings for the same resolution but use slightly different values.. switch might use timings from its own tables instead of just copying what the monitor or OS provided. I know timings does make a difference because I spent awhile in Linux manually entering timings and comparing them.