I was enchanting books on level 1 to get a Fortune enchantment to appear in the enchanting table's UI. When it finally appeared, I went xp farming to get the required 30 levels.

It took a while, but when I got the levels and went back to the enchanting table, the Fortune 3 option was gone and replaced with Feather Falling 4. The enchanting table's configuration hadn't changed, I hadn't enchanted anything in between, and when I looked at my player .dat file, the "XpSeed" field was reset to 0.

I was expecting the enchanting seed to only change when I actually enchant something.