If the sides of the bobbin aren't perfectly parallel, they won't fit right or fill on the bobbin winder, even though they'll work in the machine.
There are a lot of wobbly-sided bobbins when you buy the kind with 10 holes in them. The solid-sided ones don't seem to have that problem.
If you buy from dealers like sew-classic.co, I know that she keeps an eye on the supplies as they come in and if she catches bad batches, she sends them back to the vendors.
My Lady Kenmore runs best when I use the heavier solid sided class 15's, so I bought a bunch of them and I use them in my 15-91's now, too. Between the extra weight and the Magic Genie Bobbin Washers, they just seem a lot smoother. :)