Effective top tube length is more important on a sloping geometry - if your top tube slopes, then comparing the length of it to a bike with a top tube that's flat, doesn't make any sense... so effective top tube is more the distance from the seatpost straight over to the stem of the fork, whether or not the actual top tube follows that path.

I actually base ordering a frame on the seat tube angle & length & the top tube length... but then I know what works for me measurement wise.