Lab caches are included in the total count for paying members. Non-paying users do not have this option. Paying members must also check the following box next to this text:
Include lab cache finds where applicable Paying members only
To find this, click on the following link: https://project-gc.com/Statistics/ProfileStats and click on the Settings tab in the upper left side.
This is one of the many incentives that PGC provides along with a host of other tools, functions, and resources. As I did before purchasing a membership, you could always try the trial membership and see what those incentives are for yourself.