Top tube length varies with geometry. I hava a '89 Peugeot Montreal Express and the top tube is 60cm but dumps the bars at my knees, on my favorite bike the top tube c-to-c is 56 and I feel a little bit stretched out until I relax in to the fit.

Most actual measurements are fudged to give the buyer some idea of fit on a hypothetical standard. TT bikes measure Seat post to air in most cases due to steep seat tube angle. In that case any buyer looking for measurement x or y yould be buying a frame many sizes too large if they walked into a store with a tape measure.. It's best to just put on a sticker with a number the buyer is familiar with.