It may be that some have simply do not shift as quickly as others which were on a few days earlier so, on your visit, you find an unbalanced selection.
This lack of real choice is, though, a real bug-bear of mine. A selection of London Pride, Harveys Sussex and, say, Bombardier is, to me, not really much of a choice, and this is all too common in London and Sussex. My theory has always been that these are stocked by unimaginative landlords who've looked at the charts, decided which ones are the top-sellers and thought "if I stock 3 of the top 5, I'll sell loads and make a fortune".