Actually it's confirmed a bug:
5.3 Patch Notes
Sometimes when the skins team takes on large projects, they take a 'snapshot' of the champion's files to work on, including certain hard-coded values like base stats and movement speeds. Unfortunately, when the team finally finishes and implements the skin, those old values can be reintroduced to the game, overwriting any recent balance changes made. There have been a few cases of this happening in the past (Kayle's attack speed per level when we released Riot Kayle, Galio's Q when we released Gatekeeper Galio, etc), but rarely with so many 'reverts' to the champion. Unfortunately, Sona went through a small update during the time we were working on her skin, so she's gone back to all of her old base stats pre-champion update. While we considered shipping 5.3 with Sona disabled, we don't think the full extent of these changes merits taking her away from so many, so we're just fully communicating what's happened while we work on a bugfix as soon as possible.
After that, the very first PBE patch has her reverting her stats.
http://www.surrenderat20.net/2015/02/210-pbe-update.html#bal1