Items in Tesco (supermarkets in general) are arranged based on human behaviour (we tend to shop left to right) and based on data mining information gained from clubcards etc.
The famous example of this is the wal*mart card the USA, they found men were buying a lot of nappies on a Friday night (it was assumed the wife sent them out to stock up for the weekend) end result of this was beer being placed with the nappies, as they thought most men being sent out to purchase nappies on a Friday would like a beer.
It’s not as simple as this, sometimes they will work out the obvious most common route in the store to buy the 5 most bought items, lets say coffee, milk, bread, meat, cereals (just as examples) then you move those 5 times around so the shopper has to pass many impulse items, as if they were all in one section you wont sell much else.
Seasonal changes / social changes will over time (based on the club card information) change the order, and thus they re-arrange the store.