All the other answers are based on false Data mostly..
These odds are based on similar video poker data not real live poker ,or even online .The reason for this is sometimes,or a lot of times you have 0% whereas video poker it's always the same there are no hole cards and no burn cards which influence the unknown variables of quads and all hands in holdem poker by a lot!! Sometimes to hit the quads the card you need is burned,or usually folded or in play therefore it's not predictable at all. Same goes for a set it may be 1/8 chance to flop it or sometimes based on a 9 ring game and 3 burn cards could be 0% it's always fluctuates and this goes for flush draws,and all draws. In fact if you knew the remaining 31 cards in the deck vs other players hands you would always know what to do right? Say you got A/7 of spades in a limit game and you knew your up against KA off AQ off and jj but you somehow magically could see the remaining 31 cards and the burns then you would continue even after the flop. For instance say the flop came 9-A-8 with 2 spades vs the previous match ups I described in a good size pot,but you knew the remaining cards contained 3 sevens-2 aces and 2 eights, and to go with that the full match up of spades in the deck vs AK this is a huge beni factor for you as you got many cards to fill up,chop and 9 outs to flush out in your spade draw. Of course you never actually know this this is why Holdem poker is an incomplete data game there is no good cards in holdem especially limit holdem. As I said before you never know what could occur! Say you have kk vs 10-j and 67 with the remaining cards to be seen as 89-9 a load of 7's 10's and 2 fives all within that spectrum of the top of the deck,what makes matters worse even in the 31 random deck scenario vs these types of hands sometimes your kings aren't even in there and no running pairs to give you a counterfeit 2 pair either so your essentially dead in the water vs probable 2 pair flops like 8-10-4 -Q-5-10 etc.. It doesn't mean your one pair won't be gold it just means your hoping to hold up with no chance of improving. So to hit Quads on a flopped set of 99 all the stars have to line up right just the even have the true odds to make it in that scenario. Iv'e seen your previous posts and you enjoy 3/6 limit and play a lot you check on my post on advanced 3/6 theory gl man.