What is wrong with Card Player's Odds Calculator?

[Cohiba Al]

I've noticed that entering in two player's starting hands and calculating the preflop odds will often give slightly different answers each time you calculate them (the same two hands). Often the difference is off by .4% which might not seem like much. I was wondering why the calculator would even vary at all? How does a binary computer making a mathematical computation come up with different answers each time? Is their free calculator worth using?