As far as I can gather, the "rarity" value in mygranturismo.net is a little off the mark (I can give examples of cars they give a low rarity value for, such as the AC 427, has only turned up twice for me). The power figures are also a little vague (often don't take into account mileage/engine wear, breaking in, oil change etc) but that's another story. Still a very useful site!
Somebody in these forums figured out the "cycle" of specific cars in the UCD so you could calculate when they re-appear, but with 1.06 this apparently went out the window. Not sure if anyone has unlocked how it works since.
I do think some cars are perhaps a little rarer than others, but at the same time you have to remember the UCD at any one time shows a random selection of cars out of a possible 1000+. There are always going to be some where you are just unlucky.
For me that's the VW Nardo, only seen it once VERY early on when I couldn't afford it and not seen it since. The GT '02 I saw once a while ago, when I bought it, and once more recently. For anyone into their Ford GTs, it is a little different to later models (the back in particular looks different)