The randomization in XCOM: Enemy Unknown is what TIME's Matt Peckham has chosen to dedicate an article about, with the additional intention of trying to sway a few more people into playing with Ironman mode activated. Here's a snip:
But let’s say you’re playing through a sequence of events in a scenario that leads to one of your soldiers being flanked by a sectoid (XCOM’s “gray-skinned, black-eyed” aliens) who fires and scores a one-shot kill. If you reload, then perform each move and action in XCOM exactly as you did before from that point in the scenario, you’ll get exactly the same results.
I don’t mean sometimes or mostly, but every time and precisely the same, from misses to hits, damage meted out or incurred, where your opponents go and what they do, and so forth. Execute the same moves a dozen times in a row and you’ll have a dozen identical replays, right up to and including the sectoid shot that puts the soldier you were reloading to save down. As The Doctor might say, some events are fixed points in time: In XCOM, those fixed points become tantamount to saved games when played as re-enactions.
There’s a reason for this. Technically speaking, it’s because XCOM randomizes its numbers from a seed that’s saved at the outset of a scenario. That seed determines what happens thereafter, and once you’ve memorized a certain play sequence, if you choose to repeat it, it’s doing so in essentially non-random terms.
Bear with me here. Random numbers on computers aren’t truly random, they’re actually pseudo-random, because computers are deterministic. The best they can do is generate sequences of numbers that appear random (well, unless you’re hooking them up to geiger counters connected to radioactive material, but that’s another story). In most instances, pseudo-random is random enough. The purpose of randomization in games is to create unpredictability (the more predictable a game, the less interesting and game-like it tends to be). You might describe a game like XCOM as a predictability refinement simulation, where your choices — add this skill, equip that piece of armor, research a given alien technology — chip away at the game’s arbitrariness.