Despite the name, random number generators are not completely random.
These random number generators, especially those used in online casino games, work via a pre-programmed list of possible numerical outcomes. This is referred to as pseudo-random, as opposed to real life casinos, dice rolls and lottery tickets that are truly random.
In pseudo-random number generators, the outcome is chosen from a huge list of predetermined numbers. For example, if you were to roll a dice in a random number generator, the outcome would be the next number in the sequence rather than a completely random outcome.