History. Dates back to before the Model S was even around, when there were only 2 types - standard and performance, and the 40/60 hadn't even gone into production yet. Tesla didn't badge any of the initial cars or have designations for them in the design studio; it was either "standard" or "performance".
So people got into standard practice of distinguishing standard from performance by abbreviating it S vs. P. Then Tesla introduced their designators as they started producing the 60 kWh pack: initially "P85", "85", and "60", then P85+... until we have today's picture.
That and old habits die hard.
So the new people to the forums may not understand it - but there is rhyme and reason behind it.
(We won't get into the confusion of overloading "S" to mean "signature" versus "special signature" versus just "Model S" in addition to "standard".)