s15 seems to be skewed by a bunch of JavaScript projects including Font Awesome, which stores the bathtub icon as s15.js for some reason (note that s1 through s14 are unused, so it's not just consecutive numeric ids). https://fontawesome.com/v4.7.0/icon/bath
Plain 15 is trickier. GitHub's search is too fuzzy to see a pattern at a quick glance, but again it seems that JavaScript is the culprit with a disproportionately high number of filename:15 results compared to filename:14 or filename:16.
The only thing I’ve put together from light searching on GitHub might be to do with the fact that 15 and s15 can be used to describe school years (s for spring 15 semester) and a lot of people post assignments to GitHub.
15 is also a common number in coding problem sets that people post to GitHub. It’s a stretch but it might be something to do with 2015 being a year that has a lot of coursework commits, and 15 is a pretty low number (i.e. higher probability a student posts solutions to #1-15 than #1-19).
The same goes for schoolwork. Many people do online MIT and Stanford courses that are from previous years ~2012-2016, but there has been less time elapsed for students to post answers from 2016,17,18,19)
This is mostly conjecture, so I hope someone has a better answer!
Plain 15 is trickier. GitHub's search is too fuzzy to see a pattern at a quick glance, but again it seems that JavaScript is the culprit with a disproportionately high number of filename:15 results compared to filename:14 or filename:16.