A lot of the time things are just remastered for a cash grab by a big label so they don't actually put that much effort into the remaster. Remasters are when a mastering engineer gets something that's already been mixed and mastered and messes with it some more.
In the original mix, the mixing person gets each individual instrument track and adjusts the levels of each instrument, stereo panning, EQ/tone, compression and other effects sends so that you can hear everything properly and the right parts of the frequency spectrum of each instrument are as significant as they should be. Mastering is when you get the final mix and adjust each song as one track (as opposed to each individual instrument track) so that it sounds good regardless of how you listen to it (so regardless of how good the mix sounds in the headphones you used to mix a thing, it might sound like shit coming out of speakers, and you also need a completely different mix for vinyl because too much top end/treble are dangerous to the pressing equipment, although I can't remember why). They'll do the same kind of EQ and compression stuff, but obviously not panning or anything since that doesn't really make any sense.
So when an album gets remastered, that usually means either the original mixing and mastering jobs weren't great, and they want to do more mastering to the final masters by doing all that mastering stuff again, or it sounded fine and they want to market a rerelease in a way that will make it seem better than it really is in order to line their pockets. The latter is most often the case for X0th anniversary rereleases of albums by Nirvana or The Beatles or whatever for Record Store Day, which clog up vinyl pressing plants and make it very difficult for smaller bands to get their music out there. It's a big part of the reason that there's a 4 month waiting period for every release at every pressing plant, since the bigger labels have the money to have top pressing priority on their stupid cash grabs. Greedy fucks. If it's the former, sometimes they might still have the original stems of each individual instrument from the recording, so they could get someone to mix it from scratch again.