Originally Posted by LizaBuv
Is it the random number generator?
As others have mentioned, computers don't really have the ability to generate truly random numbers (they generate pseudo-random numbers). With 32-bit computers, you have the additional problem that there are more possible shuffles of the deck than there are numbers that the computer can represent!
I've heard that there are physical devices that you can attach to a computer that have the ability to generate truly random numbers. They involve matters of physics which, although theoretically are deterministic, in practice, they appear random.
I believe that what most sites do is pull pseudo-random numbers from a bunch of their servers in addition to random elements such as the number of milliseconds it takes 1,000 different players, for instance, to click a button. The dollar amount of the last deposit made could even factor into the equation. The idea is to throw in enough random factors and split the process over enough servers so that, practically speaking, no one could figure out what the next number in the sequence is going to be.
As to whether that decision was made 1 millisecond ago or 1 minute ago doesn't really matter so long as no one can figure out the sequence.
As someone else mentioned, they used to shuffle the remaining cards in a deck while play was still going on. While some might get very superstitious about that, technically speaking, it doesn't change a person's odds
Hope that helps. Random numbers are cool.