It's not even random for that, it'll streak the one you've done the least if they're close to each other, and if you do that one then you can be pretty sure the other one will crop up next. I'm a bit unsure if it's on a per-dungeon or a global system, but a per-dungeon system would be completely insane. Especially since things that indicate towards that (like all Human-Chemist PQI or the all Goblin Thief PQI) tend to very clearly not be choosen randomly, but streak one thing all over the map, obviously resultion in a mind-numbing reduction in variety. Because very soon you get good at it and probably insanely bored with it, and following the Chemist PQI, for example, would result in 25 Human Chemists played and not a single different one o.O
Also it results in endless cheeze-money in case you just avoid something like Orc Wizard or Orc Fighter in sub-vicious in general. Because the reason you haven't played them there is not that you're unfamilliar or uncomfortable with them but because they're imposible overkill, so you can end up with just tacking on the VT on every PQI and get heaps of cash by doing exactly the same thing as you'd be doing if you went to cheeze-grind the map. This in fact happened to me during the playthrough, made large sections of it a quite boring read because it looked like I was just spamming cheeze while I was just doing what the PQI said. And I was only really doing it to get that stuff out of my PQI hoping to get it to even out a little and give me more varied things, so it felt like a chore even though I was getting handed money (how do I get my PQI to work in a sensible way otherwise?)
But if you have to do every race/class combo everywhere before it starts goind random, that then adds... 5-7 X 15ish X 26ish runs before the PQI stops streaking? That's thousands of runs before it stops streaking stuff on you! That's what I mean when I use the harsh word insane, people burn out after way less.
And you also get good/familiar with something after 5 runs at most, streaking things past that point loses all meaning - by the time I had the Goblin thief done everwhere I was casually warmongering things Vicious with it, you'd think it's a preppable front-page strat... And I'm now actually better with Dwarf Assassins than Goblin Assassins, the PQI still ocasionally thinks it's somehow a good idea to try to challenge me with them and only ever subs them for halflings