True randomness is really really hard to do in software; bigger CPUs often have hardware random number generators that exploit some sort of quantum or otherwise non-determanistic phenomena, but in software the best you can do is pseudo-random. These are algorithms that generate a sequence of randomly distributed numbers, but in a deterministic way - from a given starting state, it will always generate the same sequence of numbers. Good algorithms are designed to make it hard to infer the starting state just by observing the sequence (if you can do that, you can run the algorithm in parallel and predict the next number), but that’s an active area of research.

At a guess, the calculator was programmed to initialise the random number generator from something that it is hard for the user to control (milliseconds since power on would be a good one) the first time you used it, but maybe TI got lazy and just initialised it to a constant value

You need to set the seed number.