Welcome to the unscientific world of graphite fly rods. Many rods are mislabeled for too light of a line weight so that they can be considered fast action. I also doubt that you did anything wrong with the build. I overline most of my rods because I prefer a softer action. You basically found the right line for the blank to suit your casting stroke. I fish a 7wt line on my custom 6wt because it casts itself with the heavier line.

You will not damage the rod by fishing the heavier line on it. The blank does not kno wif it is throwing 80 ft of 6wt line or 60 ft of 8wt line.

Enjoy the rod and be thankful that you found the right line for the rod.

Lou