I know that, in Yorkshire, (and probably elsewhere in the North) there is a line on the glass that is the pint measure. The top part of the glass is for the head. In the South, the pint is the brim of the glass. You need a very steady hand and walk to get the glass to a table or the lips without spilling any beer.
Landlords and bar staff probably know that a glass filled to the very top with beer likely means that some beer ends up on the carpet, so they nearly always serve up a slightly short measure. I suppose they think the profit is better in their pockets than on the carpet.
I suggested to our landlord that he ought to have glasses like they have in the North and fill up to the line. He said he once tried it but customers kept on asking him to top up the glass to the brim, which is more than a pint of course.
I reckon he could have overcome that by having a few notices with diagrams around the place ..but he just reverted to the southern glasses instead. A pity.