Hmmm...never noticed that before.
I would personally be inclined to say that those are reflections of the super-bright studio lights from the highly reflective nature of that chromed braid strip. I'm still wondering if it's gold or silver, though. I somehow remember the Commodore middle area being silver.
I suspect that a thorough viewing of the "you're in good hands with Starfleet" tribble-bouncing blooper scene will determine this with certainty if that bright line is moving about with the studio lighting (reflection) vs. along with the rest of the sleeve (more braid).
Re: Commodore vs Rear Admiral, the US Navy did away with the title of Commodore a while back and replaced it with Rear Admiral Lower Half. The other Rear Admiral became Rear Admiral Upper Half. Never understood that Lower/Upper Half nonsense, other than to maybe make recently promoted Captains feel better about themselves once they got to the upper echelons. So, long and short of it, it's possible that Starfleet "in universe", did the same thing after a certain period of time, post-TOS.