It's likely that being a better programmer shifts your rating scale down in general. Becoming better at anything doesn't just make you better at doing the thing - it also makes you better at seeing mistakes and areas for improvement. Like, I remember watching jazz education videos by Hal Galper, and one of the things he mentions is that there's a moment where you start feeling like everything you do is terrible, and that this means that you're getting way better because you're now hearing the things you've always disliked about your playing.
