The spam that is getting through seems like really obvious spam. Not just for a human, but for a classifier, too.
I looked at an "out of the blue" example just now:
1. Sent from a Gmail address where the name doesn't come close to matching the name suggested by the sender address.
2. Email body is in Latvian, a language I have no association with.
3. About 20 other recipients, none associated with me.
4. Subject line is nonsensical (not even words) and an email address.
5. Email body is one line.
6. PDF attachment, with no mention of it in the email body.
7. Looks automated, but came from a Gmail address.
Of course, none of these in isolation is a definitive indicator of the email being spam, but given that there's at least 7 anomalies, considering the amount of data Google has & that they pride themselves on machine learning, shouldn't they be catching something like this?