Blizzards RNG isn't as random as you'd think.

Back in TBC I must have done about 20 runs of VC trying to get Blackened Defias Armor for a hunter twink I was making.

The drops off Van Cleef were very ordered. I got 4-5 of one item, then 4-5 of another, 4-5 of yet another, and then finally I got what I wanted (and probably would have got it again a few times if I'd carried on). I was within the 5 or so runs per hour limit because of how long it was taking me to loot everything (wool selling rather well and it being worth it to pick it all up).

I've seen the same kind of pattern farming other instances, although I haven't done that many runs one after another since.

It seems Blizzard may well be using the time as part of the random seed.