This way the store can give you a "lowest price guarantee". Find it lower priced elsewhere, and we will pay you the difference!
Easy to give such a guarantee if you're 100% sure no other store sells the exact model.
Also easy to offer a lowest price guarantee when you know that customers are likely to only shop around at the high street 'competitors' which you also own. Will they bother to shop around? Doesn't really matter - heads you win, tails you win.
Although this probably works less well with much of the demographic of people who buy computers in the online shopping age.
On a similar note, when buying a scooter for my daughter, I found the "same" scooter in Toys-R-Us and Walmart. Despite identical branding and packaging, the Walmart version was of noticeably poorer quality (different materials and so on). I didn't check model numbers / SKUs, but I would expect them to be different.
I worked at a retailer who would position certain brands as junk (and they were). The junk label would be used to upsell to a better product or sell a high margin extended warranty. The pricing was usually engineered to make the warranty marginally cheaper.
The B&Ms know that people are comparison shopping, even as they browse the store: they'll be on their phones looking up model numbers to compare to online and at nearby stores. As we all know, a moat is preferable to competition, even if you have the same or inferior product.