Might be different where you live but at least in North America, a lot of places split tips between front of house and the back (cooks, cleaners etc) and they do that on a percentage of volume, not the actual tips that come in.

Say, back of house takes a standard 5%. If you tip the server 5 quid on dinner and drinks for 4 which is almost certainly over 100, the difference comes out of the waitresses pocket.

Personally, I find that a form of theft from those least able to absorb it. And being a person with empathy, I think that is wrong. But to each their own.