The most likely explanation, is that the steel used in that rod had a different alloy content, causing it to have a finer grain structure and become more reactive to the solution. Why it is different, is a quandary. Different supplier than the other rods, different batch of steel used???
Craig is correct that, at least in some places, the twist rate seems slower than the other rods. A lower twist rate can cause the lines in the pattern to be broader.
Another possibility is the number of layers used in the darker rod. It looks to be about 15 layers. The image isn't clear enough to accurately count layers in all the rods. If the other rods were made up of more layers, this would also cause the lower count rod to appear darker.
I was coming from the thought that tubes were sourced from Europe in a rough turned state.
It was my understanding (perhaps incorrect) that barrel tubes were mostly finished on the outside, and bored and chambered before they were marketed to the gun manufacturers. This was necessary in order to proof them before sale.
Having the tubes ground to near finished wall thickness for proofing, would allow for viewing the patterns to select closely matching patterns for paired tubes.
