Grouper scales

Tonight I pan-fried skin-on grouper filets. Before doing so I tried to dry the skins on the filets as much as possible by "squeegeeing" water out of them using a knife (a method I've seen described in both the French Laundry Cookbook and the Faviken cookbook). The skin looked and felt the way scaled fish skin does, and running the knife along it never pulled anything up.

But when I pan-fried them suddenly scale-like protrusions came up on the skin, and I ended up having to remove the skin (whatever they were, they weren't as hard as scales but they didn't seem to be edible either).

This was my first time cooking grouper. Anyone want to hypothesize about what happened? Are grouper scales different or what?