Interesting observation. I'd think different composition, based on your comments about finishing schedules. I wouldn't think it would be worth the effect to do intentionally, particularly since you mentioned that original finishes didn't make the distinction.
The 'twist' appears(?) less turns per inch than lighter areas. Maybe, the darker ones were from a different 'lot' of raw materials, maybe even from a different shop/forger, then stocked and used randomly when making a barrel.
The smaller strips that appear lighter, may have been worked more, possibly had more heatings to reduce to a smaller size than the dark ones. Could be, if materials started out the same, more carbon moved because the lighter ones look 'muddier', less crisp, than the darker ones.
Only thoughts, I'm probably seeing it wrong.
