Possilbly the plotter isn't precise? If that ratio holds true for all prints, then your standard 36" prints would actually be 35.9", which generally wouldn't be very noticeable. You might just notice the error when you try that really large print. From the specs, it looks like you probably spent a bundle on the thing, so I'm sure you're hoping this isn't the problem... If it is the problem, maybe you might be able to fudge it with your plotter's zoom factor, telling it to print with something like 1.0033 magnification...?
I couldn't find any reference in your printer specs as to how precise the print was. They like talking about how fast it is, its greyscale and contrast-enhancing capabilities, and they say it has 400 dpi, but that's all I saw. Maybe they sacrificed precision for speed?
