Older MTBs tended to have slack angles that "shorten" the top tube once you have the bike set up in more of a "road" configuration. The old rule of thumb is that each degree of changed seat tube or head tube angle works out to 1cm of top tube length, so if you ride a 73/73 road bike and attempt to set up a 70/72 MTB identically, you'll lose 4cm of top tube length from the MTB.