CYA due to the possibility of damage to the unit (compressor / condensor) if run under-voltage, with subsequent service calls or liability expenses?
Where was it manufactured?
While most of us (given a choice) would use higher quality equipment that could handle a 10% variance, maybe they (in the interest of saving some $ no doubt) are using components with a "fudge factor" of only 5%?
The sticker probably gives them an "out" if they're called to replace a component damaged as above. If it's in a "regular" outlet or circuit, the field serviceman can tell the owner it "wasn't installed per directions" and isn't covered under warranty service. ;(
(of course, for an additional $199, we'd be happy to repair / replace the unit...