And @ianSWBB, yes makes total sense that the "custom" geometry is just a scaling to an inbetween size. Don't know why anyone would want anything other than that really.
Ahh, maybe because the stock geometry STA is too steep to get the desired setback without going to extreme seatpost setback and clamp position on saddle rail? I'm guessing that changing STA is not allowed in the C60 custom program, but defer to ianSWBB to clarify. Unfortunately, there has been a trend in recent years to make STA very steep on smaller sized frames, and Colnago is among those trend followers.
If you can't find a Colnago size that fits without going to an extreme setback seatpost and clamp position on the saddle rails I would say one of three things is going on:
1) You're trying to fit yourself on too small a frame;
2) You're just plain fit incorrectly;
3) Your body dimensions vary so far from average that you truly do warrant a completely custom geometry, in which case you probably shouldn't be looking at a Colnago in the first place. Even their previous "custom" orders would only be done within a certain range.
As to the "trend" towards steeper seat tube angles in the smaller sizes in recent years, I haven't noticed that at all. Small frames from any manufacturer generally have steeper seat tube angles than larger frames, and that's the way it's always been. The exceptions are those manufacturers that think they can get away with basically one rear triangle dimension and produce the same seat tube angle on their smallest frames as they do on their largest frames. That philosophy makes no sense to me whatsoever. But I guess it makes manufacturing easier.