How did you come to understand that the slug coil is more often the higher wound pickup in a mismatched humbucker?
I'd think that the biggest factor in deciding which coil to wind higher would be to pick which coil contributes more to the final sound. I can think of reasons why you might want to favor the slug coil, though. A case for the slug coil in the bridge position is that it's slightly further from the bridge, where the sound produced will be louder and fuller, and in the neck position, the slug coil might be less bassy and boomy than the screw coil, since it's nearer to the bridge. The end result in either case could result in a more balanced humbucker, rather than one that is more bassy in the neck and more trebly in the bridge.