I don't think if is because of a skewed distribution or arbitrary categorizations.
I think the increase in diversity is a consequence of evolutionary processes operating in an adequately diverse environment.
The evolutionary processes operate to 'search' the available space for phenotypes. This 'search' is very, very thorough and relentless.
If the environment is diverse enough the 'search' will find more and more finally tuned things to squeeze into it.
When you consider that the environment also includes all other organisms then you can see that diversity can be driven to increase.
Only if the environment is sparse or undergoes huge disruption would this effect be reversed.
In other words, I think increasing diversity is the usual consequence of evolutionary processes.
Also, I think it will tend to curve upward as the surrounding diversity (other organisms) feeds back into the processes.