According to Know Your Meme, the PSL scale was conceived and developed on now-defunct manosphere forums, and the name abbreviates three of them: PUAhate, Sluthate and Lookism. The Conversation, reprinted by the Cobb County Courier, likewise describes PSL as an abbreviation of three defunct online incel forums. A later explanation, “Proportion, Size, Lineation”, also circulates. People post a photo and others rate it with a number. There is no single standard: the range is commonly 0 to 8 with named tiers, other communities use ten points, and the numbers do not map onto each other. Both sources call it pseudoscientific.
FaceTiers does not rate you on that scale. It measures canthal tilt, jaw angle and the facial thirds, with two more proportions, from 478 points on your photo, and turns them into a total from 1 to 8 in steps of 0.5, with one of four bands. The range looks alike; that is all. The total is our own, it is not an official PSL score, and it is not converted into PSL numbers.
Photo to photo, the same face can move by about a point on this total, because light, lens height and expression move the points the model finds. A single number should not be read as a fixed fact about a face.