Although some brands are international. in each country the company is, to some extent, independant. It's perfectly legal for a cash and carry (which is where your shop probably gets his supples) to buy goods in other EU countries, import them and sell them here.
At the cash and carry, one brand of cola sells for around �11 per case (plus VAT) when sourced from the GB company, but could be a couple of quid per case cheaper when sourced from another EU country. Would you care to guess which one a small shopkeeper is more likely to buy ?
And no, I'm not a small shopkeeper, but I do occasionally get to find out the cash and carry prices for a few things.