My interpretation, perhaps widespread in the US, is that giving anything under 15% is specifically done to ensure that the waiter understands that the service was sub-par, so the bitter feelings are ideally balanced out. Getting "really mad" or instead realizing "ah, this is because I didn't check back after bringing their food for a good 20 minutes, so that request for ketchup only came after their fries were almost gone" is their own prerogative.

If in fact they did everything by the book, especially that crucial "check in after 3 minutes / 3 bites following food delivery," and still got less than 15%, then "really mad" is probably warranted given the current situation of a sub-minimum-wage base.

I typically map it out as follows:

Less than 15% = You were nowhere to be found while my food was getting cold and I needed something for the food.

15% = Nothing terrible happened, but maybe we sat there for a loooooong time with menus closed or empty plates.

18% = Normal / everything as expected.

Greater than 18% = You rock. I asked for suggestions and you had great ones, you comped something not related to a bad screwup, etc.

Notice that all of the variation is in a pretty tight range. We could easily normalize it down about 10 points (and reinstate minimum wage) and remap the 4 levels above to <5%, 5%, 8%, >8%.