As others are stating, making a separate RNG list for drops for each character would make for more complicated programming than say "When monster dies, use RNG A, when NF is completed use RNG B, etc." Naturally, different items take up different amounts of slots on the RNG table, but it would make for noticeably more complicated programming to change it up for each character.
As for the example of "Had I logged off then, would I still have gotten Gjalla the next time I rumbled?" The answer is . . . possibly. As mentioned earlier, RNG is not truly random. Its an algorithm that uses a seed to generate a number. In other words, RNG can absolutely be predicted if you know all the variables. Bottom line though, there's no guarantee that the GHorn would have dropped the next time you played.
As for Xur, this is a bit of a different situation. For Xur, there is some sense to running his RNG in advance (saves on having to process it as thousands of players try to access him the instant he spawns each week). It seems likely (based on the ability to datamine) that he generates his list for the foreseeable future. It's also likely that every time they patch it, they can set it to rerun the RNG. (Hence why there was a slight error in the prediction last week, and the claim that he had been manipulated).
The real question comes in, can they manipulate Xur? The answer is of course they can, but doing so kind of defeats the purpose of generating a list in advance. This is why, even though Megaman was surprised that they didn't change Xur this week, I wasn't. Does it really matter if somebody data mines his stock? It doesn't make it any less random. However, the instant they do change it, they'll get even more complaints about how they are trying to fuck us over. What if they had change his list this week to sell MIDA instead of Icebreaker? First, some would argue how this is proof its not random, but more importantly, those few who do not have an IB yet for some reason would see it as Bungie screwing them over because they finally had enough coins to get it this week! In other words, its a lose-lose proposition for Bungie, so why create more work for themselves when they are going to get bitched at either way?
As an aside, its also certainly plausible that they could change the loot tables for a specific character to put it something along the lines of "The next time "Gargs454" completes a NF, give him 12 Shards." They can, technically do this, but again the question becomes why? Even if they made it to give that player an awesome item, why do it? There has to be some gain for Bungie in it and extra time programming in drops instead of programming other things (new missions for dlc, bug fixes, etc.) is, if anything, a loss for Bungie.
Just my two glimmer.