If all hardware and software is the same, you should get the same print. Which doesn't make the fingerprint globally unique, but unique enough to make you "one of these 10000 people" instead of "any of this billion people". Combine it with other data, and the combined set identifies you uniquely.
I would guess nobody uses webgl exclusively for fingerprinting, but rather, in combination with other things. Apple machines are notoriously homogeneous hardware wise. You can imagine there's a much wider spread of webgl fingerprints for android phones and non-apple desktops/laptops.
