This is all part of what some people are calling a "tip creep"
The average amount has gradually increased over the years.
Back in the 40s and 50s leaving a gratuity心づけ of 10% was the norm.
These days it's like 20% minimum. Leave a tip that's smaller than that
and you might get some nasty looks from the waitstaff接客係.
Or even, outright hostilityhはっきりとした敵意.