my totally non expert understanding is that it has to do with the type of washers - the older fibre type swell up with the warmer water and restrict the flow, and the newer ones made of ?? dont. I dont know if this is exactly true, but I can say that this phenomenon me off no end till I took myself off to the plumbing shop asked for top quality washers and replaced the buggers (in 6month old taps) and have not had that trouble since.
