I think it’s a mix of the apocryphal and the unquantifiable. Most comics hang out at the comedy clubs and have heard a lot of recycled material of people trying to make it work and stuff. The story goes that the super-prolific guys show up frequently with all-new material - most of which is gold - and then proceed to never use it again. Compare that to guys who have entire file cabinets full of jokes, each carefully crafted, rehearsed, and used multiple times and the difference becomes a bit more apparent.
Plus some guys just pump out a few stand-up specials and rest on their laurels but some guys are just super prolific. The much maligned Dane Cook has just megavolumes worth of tape in his anthology (a couple stolen alleges Louis CK). Compare that to how many recorded specials… Seinfeld has on record. Or Chris Rock.
Point is, Cosby is one of those guys who you’d sometimes hear from other comics on podcasts or whatever who shows up with brand new material from time to time. He’ll just pop into an improv and riff a few jokes like he’s planning for a special, and the jokes will never be heard duplicated ever again. To be fair though, Cosby’s got about 80 years worth of experience to draw from.