Carfax itself doesn't do anything, except check the state DMV/Secretary of state for things that were reported to the state.
Alot of states have no laws saying things have to be reported, therefor it will never show. It's the same information that you can get for free by going to your states DMV and asking. Only bonus with carfax is, they check every state for that VIN.
So things like, well, here in Michigan, no odometer readings are ever actually recorded, even though they have space for it on the title when you sign it over, that's just for the buyer and is never recorded. Therefor, michigan histories will never show odo rollbacks. Some state's allow you to not even report major accidents. Like if your car is flooded, you can simply recondition the vehicle here in Michigan, even if the insurance pays, and it's never known to the state.
Then you have the unscrupulous. Insurance companies know they can get more for thier totalled wrecked if they simply tell the buyer, who's usally a used car dealer or a junkyard, "this car has been in a wreck, it's your responsibility to either total the car and sell parts or if you rebuild it you should re-title the car under a salvage title". Yeah..how many junkyards are going to re-title the car as a salvage just because they should? 90% of "totalled" insurance deals go this way. They sell the thing to a used car lot, hand over a perfectly clear title and say hear ya go, it's your job when you transfer the title to tell the DMV it was totalled. Yeah.....