I got to 80 skill for the purpose of obtaining a specific set of traits in my horses since that's what their values added up to, aside from the fact that it has since been discovered that the combination of traits is impossible, I believe I have gone through enough foals now to comfortably say that it is also impossible for me to use the whole 80 points.
A quick search of my logs shows that from June to now I have bred just over 6k horses, many from parents that have 5 of the 6 traits I am looking for, and I don't believe a single one of them has had a total of 80 points, a whole lot of 75 and less (often much less) but never 80 so I feel like the game is not recognising the last available 5 points at least for me at 80 but I cant speak for other totals having the same issue.