A pint of pure milk

MrsW's birthday today, so we took a trip out to The Drainage Engine Museum at Prickwillow, Cambridgeshire. Don't let anyone tell you we don't know how to live it up! And before you get to calling me a cheapskate I insist this was MrsW's idea. 
Anyway, amongst the ancient engines and hand tools I noticed this Gascoigne Milk Recorder used for measuring milk, and on the dial I noticed it registered a pint of milk as weighing one and a quarter pounds. As a kid I can remember the saying "a pint of pure water weighs a pound and a quarter". My first thought was that milk would weigh more than water and then I thought about the amount of fat in milk and came to the conclusion that as fat floats in water then it is obviously lighter than water and thus milk should be lighter than water. According to Gascoigne I am wrong on both counts.