Well, to be fair, not all of them do. And some that sound lousy do so because they’re “churned & burned” pickups. That is, they’re mass produced, which means they are wound by machine and as quickly as possible.
But the issue I’m referring to is coil shape and coil size.
Here’s a pickup that comes in a Japanese Jazzmaster.
Notice how tall the coil is. It measures 1/4″, the same as a humbucker or P90 bobbin. That’s a lot of space between the bottom and top of the flatwork and is far from vintage.
Here’s one of my Jazzmaster pickups.
This is vintage correct spacing at 1/8″. Vintage Fender spacing seems to have changed some from year to year and I’ve seen CBS era pickups with 7/32″ spacing.
Coil shape has a lot to do with the tone of the pickup and this has never been more true than in a Jazzmaster, where the coil is very short and wide. And wile there are many other factors, like handwound vs machine wound, coil shape is a paramount factor.
When you have a Jazzmaster that doesn’t sound right, this is a good place to look. I’ve replaced tall Jazzmaster pickups in a lot of different Jazzmasters so it’s hard to say whether you have them or don’t without looking. But the difference in tone between the two is drastic.