It used to be more consistent coz i kept using the same location (my rooftop) so the GPS is rather constant, i measured 1 round and GPS was within 10 metres vs measured (assuming measurement quite accurate, which probably is) ...... but yea you are correct nowadays it's gonna more variable if doing in various locations.
Anyway i don't think fitness has improved (or worse declined) significantly.......but just by doing more "time trials", i guess just be sheer experience/pacing it can improve. Like the 10K, 3K "TTs", same route start and end point.....the timings jumped from 41.xx down to 39.xx and 11.4x to 11 flat. Just by sheer number of tries/luck.

ie I don't dare to claim the distance is accurate but it usually is the same start/finish area then there is still some degree of consistency to it
1 year if wait for only what...2xu, AHM, SCM high profile, it's only 3 times. No off season whatsoever.
Then the cheaper races, they are probably worse off in distance accuracy than my GT Runner's GPS. Like the Skecher's last year.