Honestly I think it’s just insecurity, and maybe in some cases a desire to get their “money’s worth”.
No one wants to pay a few hundred dollars to go to a clinic to work on 20m circles (even if that’s what they really should be working on!). They think “oh I can work on that on my own”, even though they clearly don’t, and instead want to work on the things they’re less sure of doing on their own.
I used to wrangle at a dude ranch and on the little questionnaire we gave people on their first day, nearly everyone marks themselves as “total beginner” or “advanced/expert” rider. And we always put both on “beginner” horses because if you asked, every single one of the “expert” riders would admit that the reason for marking themselves as such was because they had gone on 10 whole trail rides and “run” (not trotted, loped or galloped - run) a horse before. It was such a perfect representation of the Dunning Kruger effect, it became a running joke for us. Interestingly, the people who wrote down “intermediate” were usually the actual “horse-people” who were probably riding several times/week back home.
In any case, I usually refrain from commenting on my own riding as much as possible for exactly the reasons you point out. The trainer/clinician is going to SEE that you’ve oversold yourself
I’ve probably said this before, but I find it’s always better to follow the Tyrion Lannister approach that “anyone who must say ‘I am the king’ is no true king”. If you’re amazing, people will notice - and if you’re not… well, at least you can spare yourself some embarrassment and a few snickers at the rail.