...It should be noted that TPTB went cheap on these insignia, probably realizing that they would be too small and dark to recognize on the television screen. They only ever used three variants:
-A single dark stripe for most personnel, regardless of whether they were called Ensign or Crewman
-A single dark stripe and two bright ones (presumably Lieutenant Commander, to match the *oo regular Starfleet insignia for that rank) for Chakotay
-Two stripes, usually one black but occasionally both bright, sometimes perhaps even both black (presumably Lieutenant junior grade or Lieutenant, *o and oo in regular Starfleet respectively) for Torres
I don't know why they even bothered with Torres' insignia, but it does look as if they hand-painted it, differently from episode to episode...
We could ascribe the changes between black and bright to the glare of our TV sets even when it's in fact real costuming. That way, the single stripe could legitimately mean Crewman when dark, and Ensign (as with Seska) when bright, even though Seska's stripe in reality was always dark.
Timo Saloniemi