Apple can allocate money to OS X development independently of hardware sales, but HP is committed to paying Microsoft a fixed fee per unit sold. That's the difference. It's possible that HP is actually paying Microsoft more money than Apple spends on pure OS X development.

I've heard that companies like HP spend more than $600M a year on Windows licenses, and that's at OEM wholesale prices, simply because of volume. From every bargain-bin netbook to high-end server, they get taxed.

Although Apple doesn't disclose their budget for developing OS X, the core development team is surprisingly small in relative terms. Apple doesn't have to cover as many bases as Microsoft does.

Every so often you can read of grumblings about how much some OEMs pay to Microsoft and how they could be better off keeping the money and doing it themselves: http://www.electronista.com/articles/08/09/12/hp.developing....