I had been having trouble becasue of this very fact (am staying April 20-24th) - on the website the marriott's only availability was at 160GBP reducing to 119GBP on the monday night, room only. called hotel late last night, hotel reservations closed so got through to the central reservations in the States who gave me price of 110GBP for 1st 3 nights and 119GBP for 4th night.
With regards to the Thistle, i called them directly also. they were offering b&b rate at 130GBP per night for the 4 nights. i queried the reason for the same rate for sun/mon night as fri/sat night and they immediately reduced rate for the last 2 nights to 95GBP per night!
Confused, i know i was. Also, found that a lot of hotels had no availability at all according to the website, but did when i called them!