Okay, I get it. You are right, if I don't set the OS X system settings to 'Scaled' then it does offer 2880x1800.
But for my usage, I need the OS X setting to be Scaled to 'More Space', e.g. 'lookalike 1920x1200'.
So it makes sense now. It doesn't look super great in Windows 8, but with the guest OS scaling set to 250% it looks pretty decent. I can live with it. Much better than Fusion 5, to be sure.
Thanks for your help!