Yes, it can be a real diamond mine - yesterday I got Tea for Two for £25! And there were several Lutens (not as well Discounted). The website does have a 'how we do it' section that mentions manufacturers overproducing / old season stock / stores over ordering etc.
With fragrances some are clearly cosmetally damaged - I guess the brands would rather ship them off to a discounter rather than have less pristeen stock on their shelves. Some are probably returns that they feel can't be sold at their exclusive stores.
With brands like L'artisan I suppose a store will re-order (from the manufacturer), say TFT when stock gets low e.g down to the last 5, when the new stock arrives they may ship off the residual stock therefore keeping a consistently new batch within the store.
I hear creed has an annual bonfire rather than risking its brand exclusivity by gracing the shelves of T.KMaxx!