I don't think making items from cloth is an exact science. My Essential Carryon is supposed to weigh 1# 12oz but weighs 1# 13oz. Maybe the cloth they made your bag from was just a little thinner than mid-point thickness. But hey, that's 7 less ounces to carry or 7 more ounces you get to put in it. I only carry on, and I handle the bag carefully, so I don't need a bullet proof bag. As far as I'm concerned most bags are too heavy.
What really irks me is the way bag manufacturers promote the bag volume. 22 inches x 14 inches x 9 inches (regulation carryon size) is 2772 ci, but they usually quote more volume than that. At the volume they say their bag can hold, the sides must bulge, and so it wouldn't fit in a sizing template. Add to that rounded corners, which subtracts from volume, and 2772 is the external volume. Nylon itself occupies more than 1½ ci per ounce, so the heavier the bag, the less the internal volume.