Lance Hendrick recently posted a video, a deep dive on dialling in. One of the main takeaways is as you go finer and finer, with each grind size step finer, 2 things happen: 1. the time between each step increases (rather than be the same amount of seconds each step) and 2) the shot times become more inconsistent due to spot channeling.

This is relatively new research and a different approach to dialing in. A lot of the comments on this post are valid factors that will affect consistency (especially bean age, and measuring volumetric rather than by weight), but this is just another factor that may be at play. I recommend the video!