Glad I could clear that up for you guys! (Funny story I've actually been trying to reg here for months but the forums didn't like any of my e-mail addresses...)
Yeah, it basically comes down to the genga artist's preference/ability most of the time. Sometimes they don't even draw in any of the details for the genitals and leave it to the colorists to figure out. You have to remember that, for most of these companies, the idea of an uncensored western release is the furthest thing from their minds, so why spend the time drawing something that no one's ever going to see, right? There are also some coloring conventions used under mosaics specifically to give a certain look with the mosaic on (exaggerated colors or contours to give the illusion of detail through the censoring). Sometimes there are even silly things under the mosaics (iirc JAST had to have some of Demonbane's CGs retouched because a few of them had smiley faces drawn on the penises under the mosaics!).
There are some artists who are just really passionate about drawing porn and put a lot of detail in even if it might never be uncensored––one of ClockUp's artists, Hamashima Shigeo, says that she likes to put a lot of detail beneath the mosaics on the off chance that someone will get to see it. But not every eroge artist is particularly passionate about porn, so you end up with quite a variety of detail levels and whatnot. Imouto Paradise's animations, for example, were completely messed up beneath the mosaics (dicks clipping through legs and all sorts of weirdness).