The machines they use are pretty damned accurate, I have sent some bottles of my home brew in with the old man and he gives me the print outs back with calories, ABV, colour, turbidity (no IBU :-( ) etc..
Basically my last brew was exactly 5.690 percent by volume, if they can tell that far beyond the decimal, I'm sure whatever it is, they are hitting targets and getting pumped if they vary to far out (my assumptions only, don't quote me).
Regarding my reading, I'd say that mine would be labeled 5.7%, not 5.8 or 5.5 even though .3 may be allowed. Again, just assumptions. At 5.69, they could really say 5.99 and still be within spec?!
Seems ludicrous! maybe that could be a legal loophole for DUI haha