The top tube isn't parallel to the ground; instead it slopes downards a bit. I think the difference between "semi-compact" and "compact" is how extreme the sloping is.
Compact Frame:
Traditional Frame:
I'm not convinced that one is clearly "better" than the other ... apart from aesthetics, the main issues that I see are that a compact frame allows you more standover height room (which may be good if you are short, or oddly proportioned), but you might not be able to use a full-size drink bottle in the seat tube-mounted bottle holder.