That's the thing isn't it. You can try to "standardise" a teaspoon, but when a fat person is making tea do you think they put in 5 heaped teaspoons? or 10 levelled ones? or somewhere in between?
You can try to standardise a teaspoon, but in practice it's very hard to actually fill a teaspoon with the standard amount.
A "teaspoon" in practice is relative to the individual.
It doesn't matter, it's a relative amount to help with context. Again, most people would have a pretty good idea how much a teaspoon of sugar is - but not necessarily how much 23.7g of sugar is.