First off, Rocky, I've used a lot of Fakro windows. They're a very good product.
As for pricing. The price of 2 x £2100 in the catalogue may well include VAT. I've trawled through it but can't see any reference to VAT. Advertising for non-trade customers usually has to include VAT.
The £2538.34 price is aimed at trade......... and of course to pointlessly confuse the punters into thinking it's cheaper than it is.
Only guessing, but it looks like £4200 versus £3046.
It probably comes down to the utterly stupid practice of giving a "list price" which is inflated, if not totally fictional. Then they give a massive discount ..... and you arrive at the "proper" price eventually.
It doesn't do anyone any favours. At my Electrical Merchants, they list ridiculous prices which no one would ever believe, then we all get 85% discount. What's that all about?