This is a bit disappointing (especially since I have one on order), but not entirely surprising.
Focal lengths are routinely rounded up (and often times down on wide angles). The 70-300L is noticeably shorter at the long end than the 300L F4 prime. It's quite obvious.
It's disappointing but, I suppose as long as the 100-400 II is 100mm longer than the 70-300L, then that makes it worthwhile.
This is terrible, perhaps I should send mine back. It's clearly dreadful that the lens shortens its focal length as it focusses from infinity down to less than a metre, just to obey the lensmaker's equation and keep the length of the lens constant. You would have thought that the Canon engineers could have the the lens gradually lengthening by about 500 mm as you focussed down to keep the focal length at a constant 400mm. Perhaps the Mk III will do that.
Seriously, in order for the lens to focus down to 0.98 m and have a magnification of 0.31, the focal length drops to 304mm. It's a small price to pay.