The short answer is: She is a liberal Republican (many would say Republican In Name Only or RINO) and her opponent (Jerry Brown) may very well get the majority of votes from a traditionally conservative group, namely gun owners.
Whitman's positions on many issues (gay rights, abortion, the environment) are in tune with more conservative Democrats. Even when she expresses support of conservative causes, many conservatives find it hard to trust her. Her association with Barbra Boxer - and the $4,000.00 she gave her campaign - do not help her win the trust of the Republican base.California's gun owners are probably at the top of the list of people who do not trust the Republican candidate for Governor. While she headed eBay, she instituted a policy that went much farther than forbidding gun sales - it banned the sale of most gun parts and many gun accessories - ALL OF WHICH WERE PERFECTLY LEGAL TO SELL VIA MAIL IN ALL 50 STATES. For those gun related items that still are permitted (i.e. holsters) not even a picture of a gun is permitted. Paypal (an eBay company) began suspending accounts if funds were used to purchase any "gun related" items. As one might expect, "gun rights friendly" replacements have since opened shop on the internet - but to this day, many gun owners refuse to do any business with eBay. Although now that she is running for governor, she says she doesn't think any new gun laws are needed "at this time", she has snubbed several gun rights groups by refusing to meet with them. Enter Jerry Brown, arch liberal and current Attorney General. In this office he is charged with enforcement of California's many firearms laws. One would expect that he would have been a thorn in the side of California's gun owners - but nothing could be further from the truth. When he took office "Office of Firearms" was hostile to gun rights - even going so far as to issue regulations with no support in law. One of the first things he did was to knock the firearms department down to a bureau from an office. Shortly thereafter several of the employees responsible for worst of the abuses no longer worked there. Brown then set the new "bureau" to work tracking down convicted felons and domestic abusers who failed to dispose of their firearms after their convictions. Trust me, this is the kind of enforcement law abiding gun owners like. Brown won more points with California gun owners by writing a brief to the Supreme Court supporting the application of the 2nd Amendment right to keep and bear arms against the states. In it Brown stated that such application (called incorporation) was necessary because the California's constitution provided no such protection. It would seem that, like Howard Dean, Jerry Brown believes in the entire Bill of Rights.
Add to all of the above the fact that our current Republican Governor has angered gun owners by signing a controversial ammunition law, and it is very doubtful that Whitman will get many votes from gun owners. In fact the NRA, which cares about only one issue - gun rights - may very well endorse Brown, just has they have done with in the past with Howard Dean and Harry Read. If Brown does win the NRA endorsement, it could mean a swing of as much as 5% of the vote - and that is more than enough to swing any election that is even remotely close.
Yes, Virgina politics does indeed make for very strange bedfellows.