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.