AITAH for short-tipping a waitress?

I tipped a waitress 15%, instead of the current minimum standard of 20%.

When my wife and I dine out, we always tip 20%, or more for excellent service.

We go to a local diner regularly, where all of the waitresses and waiters are great, and the people who buss tables are good hardworking people.

Recently, we had dinner at that diner, and had a waitress that we had not seen before. Her service was subpar, but not bad. She got our drink and food orders right, but didn’t come back to check on us, even though there were few customers, and we had to ask someone else for additional napkins because she wasn’t around.

I intended to still leave her 20%, because I would usually only go below that minimum if a wait person is particularly bad – I’m usually accepting that any person may just be having a bad day.

But, then, while waiting for the check, I overheard this waitress complaining about another employee. She was talking to another waitress well within earshot, complaining that the guy who busses tables doesn’t clear her tables fast enough, and she said that she doesn’t understand why he’s so slow, because she gives him “the standard 15%”, which she made sound like a generous from her. The guy she was complaining about has worked there for a several years, is hard working, and gets along well with all the other waitresses and waiters.

I considered her complaining about another employee within earshot of customers to be very unprofessional, and still would have most likely tipped her the new minimum standard of 20%.

But what bothered me most, is that the standard customer tip to wait staff increased from 15% to 20% decades ago, and I thought that the wait staff percentage to buss staff would have also increased to 20%, the new standard, but was surprised to hear that this waitress is still tipping bussing employees the decades old 15%, and is complaining about them not working hard enough for her for that 15%.

So, I tipped her “the standard 15%”.

AITA?