I read it as that too and sounds about right really.. I have one of these very similar but not same make and it's very slightly off. Could just be the thickness of the ruler itself, the metal is quite thick around 3mm, the way the ruler screws into the square at the end makes a difference too, if it's not screwed up real right the square will be way off. These make great tools for quick jobs that don't need to be 100% accurate.. it's basically a multi tool: has a ruler, square, spirit level and a carpenters mark (little brass screw in spike on the end for making quick marks) all work fine for a begginer but if you need accuracy then buy separate tools!
I think @cloudwarrior means the length of the rule was out by .75mm but that the square is actually 90 degrees - I may have misunderstood though - so I was asking what he'd checked the 300mm against not the 90 degrees. Some reviews on amazon say it's well out of square (too) though which has put me off. I could live with the length being 0.25% out but not the square being 2 degrees out.
The beauty is you use the square itself to measure it's accuracy. Much like how you'd test a spirit levels accuracy is to put it on something level and spin it 180 degrees horizontally to check it reads the same both ways. With the square, put it up against a straight edge (sheet of timber for example) and with a pencil, mark the perpendicular. Flip the square over so the pencil mark is on the other side of the square. If the pencil mark is exactly the same for both, it's accurate. If the pencil mark runs out either way, it's not square. Happy testing!
Some pretty bad reviews on amazon for this being well out of square.
Hmm I could probably live with that kind of tolerance on this kind of tool at this price. What were you comparing it to for accuracy?