Strange experience with a server - is a 15% tip insulting?

I am visiting from Germany, and went out to a nice sushi restaurant last night. Waitress was very nice and helpful in deciding what to get.

At the end of the meal I tipped 15% which is extremely generous back home. (And on a $500 meal for my friend and it meant $75 for bringing a few plates!!)

She didn't even look me in the eye and barely whispered "thanks" before walking away.

I don't fully understand what happened here. I want to go back to this place next time I visit but not sure if I feel welcome after this.

Now I am wondering if servers don't get a base salary and only rely on tips. But even in this case - she would have made maybe $300 that night from the other tables plus mine (if I assume people do 10%) so it doesn't make sense why she would be so angry.