They do "correspond to" and "are really" some other distance. It's like saying Kerbin doesn't "correspond to" or "[is] really" Earth, or that Duna doesn't "correspond to" or "[is] really" Mars. It's pretty clear that it is. Alternately: Is Kerbin really a 600km planet that ignores real material densities? Alternately2: By the way, I'm not saying that the planets are LITERALLY only 6,000 km or LITERALLY only 600 km. I'm saying that it's useful to think of them as 600 km when discussing physics as it applies ingame, and useful to think of them as 6000 km when discussing hypothetical planetary science. ExampleToIllustrateAlternately2: If you want to talk about an orbit around Kerbin, you have to think about it as 600 km or you get a nonsensical description of an orbit within the surface of a planet. If you want to talk about the internal structure of Kerbin, you have to think about it as 6,000 km or you get a nonsensical description of a 600 km asteroid that should have cooled and lost its atmosphere eons ago.