Google gets resumes and random text from 1000s of people a day looking for a job, all over the world, completely bereft of any sort of qualification. That's the vast majority of resumes any large business like Google will see. It's like Spam writ large.

So we have resume screeners who look at those random resumes and try to separate wheat from chaff. For some, we have a phone screen that is literally to check the most basic of facts to see if the resume is at all consistent with the name at the top, then a real phone screen with an engineer, then an onsite, then offer.

Each step loses people, but the first, separating the clear bs from the real is where the huge gap comes from.

That said, I hope we have more false negatives than false positives every step of the way, and for that, all I can say is that I wish it didn't have to be that way. Well, that, and this: Get referred by a Googler, skip many of the above steps :-)