Err...nothing, the way you state it. I think you didn't quite mean what you meant to mean. Hope this is not logical quibbling.
I think something can be quasi-random but still doable, and I think that's useful if people are trying to learn patterns, or if you want to give credit for rewarding patterns so nothing TOO unfair happens when you put random stuff together, e.g. if you had no way out after having to shoot a bunch of enemies because it took too long before refueling.
I suppose you could also have, say, a sequence of 1000000 random numbers, subject to the first 100 being from 1 to 100, up to 1000 being 1 to 90, up to 10000 being 1 to 80, and so forth. This would still be random, but repetitive, to allow for pattern recognition etc.
level 1 = 1st 5
level 2 = 1st 10 (as opposed to 6-10)
level 3 = 1st 15 (as opposed to 11-15)
...and so forth.
It would still be random, but a bit hedged. In either case, we could claim conscious design. Just in one case, we would deliberately want to repeat, and in the other, we would not.
Hope I'm making sense.
My principal said, 'Emo, Emo, Emo.'
I said 'I'm the one in the middle, you lousy drunk!'
-- Emo Phillips