Even in the days of level top tubes, the measurement could have different meanings, depending on the manufacturer's preferred method of measuring. Some measured center-to-center, which was from the center of the bottom bracket to the centerline of the top tube where it met the seat lug (the intersection of the centerlines of the top tube and the seat tube). Others measured center-to-top, which measured from the same bottom bracket centerline to the top of the top tube at the seat lug. Now, with sloping top tubes, the numbers (IMO) are all suspect and have to be examined individually on a bike-by-bike basis.