Most smaller sign shops have low end plotters. The machines are simply too expensive for the Mom & Pops to keep up.
They probably had a 300 dpi plotter. Depending on their RIP software, the results will vary.
We use HP DesignJet 5000 ps's and DesignJet 3500's. Both the 3500 and 5000 are capable of traditional screening as well as stochastic screening. The 3500 will do 300-600 dpi and the 5000 will do up to 1200 on special glossy paper.
My guess is the sign shop used traditional dots for your bezel (maybe their RIP and / or plotter doesnt support other methods of screening) Most, if not all new(er) plotters use the stochastic screening process. See below for a sample of the two:

OTOH, the sign shops plotter may be a top of the line machine designed to make billboard type displays. Ever seen a billboard up close? The dots are like tennis balls! Those type of machines, geared towards billboards and the like, typically poop on smaller signs designed for close view.
My $.02
-Tom