It looks like Anki is applying a small amount of fuzz:
https://docs.ankiweb.net/studying.html?#fuzz-factor
My understanding is that fuzz is only applied to review cards’ intervals, not to new / learning / relearning cards’ steps, but it actually seems that the program applies a small amount of fuzz also for learning steps, even if the buttons don’t reflect that. Of course I may be missing something, maybe @dae or somebody else can confirm this.