That is odd indeed. Playing the devil's advocate, I would wonder if that there was major damage that required new fiberglass and new gel coat.If so, the gel coat may have been improperly applied.
Second case could be that the inside was sprayed with some sort of coating. In that case, the question is Why and is the problem going to spread over time.
The worst case that I mentioned needs to be ruled out. Getting to the back of that area would help a lot but I understand that having a look my not be easy. I would suggest that you look very closely at the outside of the hull in that general are to look for any signs of repairs such as wavey glass, ridges, uneven gel coat etc. In my typical doom and gloom persona, I have visions of something piercing the outer hull right into the deck area.
The advice to have a survey dome is spot on if you are interested in the boat.
The issue could be something simple like a bad paint job. You just need to know more.
I hope it is something simple.