AutoCAD Inventor :: Keyway Generator Giving Incorrect Sizes?

I am using the keyway generator to place a keyway in a hub and shaft, but the depth of the keyway (and the height of the associated key) is not right.

I am using an 18mm and a 20mm dia shaft, both of which should use a 6mm wide by 6mm high key according to the ISO standard. [URL]

But Inventor gives me a 6mm wide x 4mm high key and the keyways to fit; so if I use a standard 6mm high key in real life it won't fit. There doesn't seem to be any way to change to a custom key height.

Or is Inventor using a different set of standards? It's pretty good at matching standards in other places, for example the gear and fastener generators, so this seems strange to me.