I'm completely confused by Gmail's spam filter. We've already discussed its extreme tendency to false positives, but it's quite good about false negatives - no

I'm completely confused by Gmail's spam filter. We've already discussed its extreme tendency to false positives, but it's quite good about false negatives - no spam hits my inbox. Except... Google claims >90% of spam never even reaches your Spam box, only the ambiguous messages. So why do I get 10x the spam at Gmail as at any other account? Are they not actually good at detection? Are they just a high value target that gets hit more often? Or (my guess) are they trying to get clever? Google Doc's spellcheck is infamously bad because they're trying to use machine learning approaches without having a simple English dictionary as a fallback. Is Gmail trying to detect solely on email sender/content and totally abandoning approaches like greylisting?