Sometimes, pricing models aren’t as simple as you’d think. Halfords aren’t necessarily competing directly with CRC etc- with the whole internet to choose from, you end up in a race to the bottom price wise, and also in a sensitive market product wise. All of which means shifting volume, reasonably quickly (you need to deal with a lot of items to get a decent sized profit and you need to sell them quick to make sure you don’t get stuck with millions of 2011-spec parts in 2012)
So, instead of going for that market, they go for the “aaargh my son’s bike has broken sell me whatever I need” market, in which you sell less items but for massively more. Carry less stock, make enough off the sales you make to cover the sales you don’t.. It’s a less effective approach generally but that doesn’t mean it’s bad, because it’s a less effective approach operating in a slightly different market to the more effective approaches.