That's because the UK uses imperial pints, a US pint is only 473 ml, and the "shaker pint" is only 414.

To make it even more fun there are US barr associations trying to claim that a pint is just a glass of beer not of any specific size, so they can use the word with a smaller glass. Fortunately most government weights and measures agencies are not exactly convinced that a pints means something different than the standard measure.