I have made that comment in several places, and probably here as well. It is a well known saying in the modelling/simulation community.
The discrepancy between HM and FF is a good example. The discrepancy between indoors and outdoor performance for some people might be another. You can see it here in the way that some workouts are easier for some than others. The Omnium (which I have not done in a while) is one of my favorite workouts, for some, it is a horror show.
I do 4DP testing (HM and FF) because it helps calibrate the workouts. I do not assume it tells me too much about exactly how I will do an outside course.