OK, I had my morning coffee now, let me try again.
1. You said it was custom - who built it?
2. Why do you think the top tube slopes forward? Seems to me that the only way to really tell would be to put the
frame in an orientation where the head tube and/or seat tube were around 72 degrees, and then check the level of the top tube.
In addition to my wheel suggestion, it occurs to me that even if the wheels
are the same size, if the fork is too short (below the head tube), the same effect would be seen. Therefore point 2 above.