Nothing. It's the choices that Walmart makes that makes the difference. You will never see a TOL machine there, and you will never see a top-quality one there, because they go for selling things for low prices.
If it's a higher model, it's a better quality.
Most of the sewing machine companies these days don't have uniform quality through their entire model line as they used to do 30 years ago. The low-end machines are not good quality. The mid-range and top-level machines are good quality. With that said, there are exceptions everywhere in individual machines, I would suppose.
Seeing as though Walmart is likely to ask the manufacturer to issue a unique model number for their store, as many other mass merchandisers do, you may never get proof that way.
We bought my son a lower-end White machine for $250 and it wouldn't hold its tension more than a week or two. If you had a Singer that broke its gears, it could likelyi have been a cheap, made by the lowest bidder Singer machine. My 50-year-old Singers are still going strong.
Maybe you aren't using them for things as heavy as what we do -- denim, etc.