I'm no expert, my sinaloan is just a baby still. I was thinking that it might just be a little chubby and showing some skin under the scales, but that really doesn't look like it's the case.

I'd say that it is an inheritable trait though. The only other thing I could think of if is its growth patterns or something that would affect it one way or another...

it does just look like the tips of all the scales have a little black in them, which is how mine looks. It might just be a normal trait for sinaloans.
Hope I helped?