No, it’s simpler than that. It’s just the number of different species found as the amount of time spent searching increases.
Think of it as analogous to the number of different flavours found as you pick jelly beans out of a bag. Initially you’ll get a new flavour with every bean. After a while, some of the beans you pick will be flavours you’ve already encountered. Eventually, most of the beans you pick will be flavours you’ve already encountered, and it’ll become rarer for you to pick out a bean with a new flavour.
It’s a bit more complex for fossils since they’re not distributed randomly but tend to be found in clumps of the same species. But there’s no way that you’d expect to see a curve like Bechly’s, which indicates that at the midpoint of your exploration you’re finding lots of new species with no effort at all.