Its not particularly "hard" to get the proportions right, it just takes more and more time the more accurate you want your graphics.
If we take GT5P (or Gran Turismo in general) as the peak, I'd say that Ferrari Challenege is very close, almost spot on.
There aren't any tracks to compare the two unfortunately, but I can compare Monza, Silverstone, Hockenheim and Montreal (Redwood Park) in F1:CE and they seem to be very similar, a few proportions of corners slightly different here and there.
The difference between these two games is because both these developers probably didn't take laser scans of the tracks, they probably at most went personally and took many, many pictures. So, when your level designer sits down to draw out the track in the game engine, their measurements and proportions may be slightly different - influenced from lots of things like angles of the photos, shading, the relative sizes in the game engine, etc.
For example, if I drive around Hockenheim in F1:CE and FC, I find that turn 3 is ever so slightly different in its angle and relative position from turn 2. Having never driven around the real track, I can't say which is closer, but there is an obvious (to me) difference.
I doubt its intentional, simply take it as the developers interpretation of what the circuit is like.