Based on a large dataset of drug-free powerlifters competing without equipment at regional, national and international meets. The equations used are from article Relative strength - Wilks, IPF GL and allometry.
The calculator is most useful as a relative comparison between lifters with different characteristics. It is less useful as an absolute measure of strength, but can be used that way too. Note that a 20 percentile is quite impressive if you're a typical gym-goer, since you are compared against powerlifters who compete.
See also the ExRx.net Weightlifting Performance Standards and strengthlevel.com.