Several examples:  << no Repub observers,  << few Repub, but many Dem observers  << boundary fences so Repub observers are so far away they can't really observe, << there's back rooms & loading docks for ballots arriving where observers are not allowed or not there constantly watching  << there's court decisions on allowing observing not being implemented & with sheriffs refusing to enforce compliance.   On & on. 

 If you want to search & see about observers at this Detroit location, I'd be interested in what you found.