My take is that the 0.7° top tube slope wasn't nearly as much of a concern to the framebuilder (Dave Wages, BTW) as getting the HTA and STA relative to the dropout centerline datum right, so it wound up where it is in spite of the identical angles shown on the drawing at both the baseline and the top tube. This is the reason I don't think it's a good idea to use the top tube as a datum.