So if you go to the multiplayer window in 1.6.4, and have a server added that has been updated to 1.7.2, it will show as 1.3. I attached a screenshot of the 1.6.4 screen. Apparently you can't take screenshots at the main screens so I recorded a video. It is unlisted on my channel at http://www.youtube.com/watch?v=cAHbZ_8lZ68. This issue also happened with 1.6 servers with 1.5.1 clients.