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 (when caterpillars receive less sunlight and temps start to cool, it's possible the dark bands are thicker to absorb more heat to speed up metamporphosis)

2. Autoimmune response to pathogens- If you're concerned the caterpillar is sick, you can always separate it from others and place it in a separate cage or at least separate on its own milkweed cutting. 

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 these caterpillars. Here's one case I documented a few years ago:

Dark Caterpillars...did they survive?