The difference between US/UK weights issue price was subject to current exchange rates, cost of shipping and the wholsesalers pricing policy - naturally it was loaded on the high side and was probably also based on marketability. As I add items to the SG cats I include the UK price list, where available, and this shows increases and decreases in prices for long running designs that can only have been arbitrary decisions.
Pre internet US imported items to the UK used (apparently not factually) an exchange rate of only $1 to the pound, making the items relatively more expensive in the UK. This types of price manipulations are a fairly normal business practise. Government tariffs also have an effect.
I would expect that the valuations given are based on B&M auction results in the period 1995-2003. This is the reality of price guides, they are all based on a distorted marketplace which the second-use market always has and always will. When I was collecting seriously, I would pay as much as it took to secure pieces I wanted in auction and was only beaten if someone richer bid against me. Ysart paperweights reached there peak when Parkington was collecting he rarely put a bid limit on - I sometimes bid for him and the instruction was to win. Sometimes we unknowingly competed as one of his other bidders had the instruction and glassware prices rocketed, also there was a third deep pocket bidding on Monart in the late 80's and he would never make an arrangement. eBay has helped prices reach more realistic levels as scarcity is more easily judged - lok what it did for Avon collectables that had sustained steady increases in value based on price guides - the bottom fell out the market when people realised that these were so widely available. Now only the rarely seen Avon's fetch reasonable money. The same applies to most mass produced glass with the notable exception of Lalique, but in that case here is a huge and wealthy collector base. People seem to forget that pressed glass was produced in huge quantities and if an item was made in the last 50-60 years there is no way to assess scarcity as most examples are still owned by the original buyers. Even really scarce Vasart glass has become a lot more available since 2000.