Mitt $$ Spin
There's a difference between raising $5M for the primary and raising $3.5M for the general and $1.5M for the primary.
For one thing -- you can return the unused general election money if you don't make it to the nomination.
So -- and I'm not trying to be smarmy marmy here, it's probably fairly easy for Romney's supporters to fork up a donation they anticipate getting back.