Unusually thin or thick black stripes on caterpillars?

There is a natural variation of stripe thickness on caterpillars, but what about when caterpillars have unusually thin or thick black stripes?

To this point, there are only theories as to why this occurs including:

1. the amount of sunlight they receive

2. Autoimmune response to pathogens

I've rarely seen this over the years, but it seems to be more common with the migratory generation in late summer/early fall. I've never noticed  any health issues with dark or light caterpillars. Here's one case I documented a few years ago:

Dark Caterpillars...did they survive?