http://en.wikipedia.org/wiki/Polaris_(poker_bot)
Hands in limit poker nearly always have a mathematically correct way to play; if a program executes the correct play every time and isn't figured out as a bot the program stands a pretty good chance of being profitable.
Most human players don't have the advantage of being able to memorize the correct action for every scenario, a well written bot has an absolute advantage with regards to the math involved.
Additionally, a poker bot remembers every hand it's ever played, as well as every hand it's ever seen played - there has been poker software available for many years that will analyze other player's play for weaknesses which can be trivially exploited. I would assume diligent poker bot creators know about this type of thing and are taking advantage of that as well.