It's happened to me and I'm sure of that, because this day with an event one of my villagers became master doctor at 100%. I tried some stews that make the villagers healthier and, after eating one he was still master doctor but at around 90%! The other villagers also had their skills decreased a little. But it seems that some other skills had increased a little in a random manner (not absolutely sure)! I think it was a stew with the "Red Anthurium" near the dam (3 on the keyboard) inside, but I don't know which other herbs.