That could well be. It's hard to know without access to the data, but they could probably do better if they tried. However, I fear if Google were to try harder

That could well be. It's hard to know without access to the data, but they could probably do better if they tried. However, I fear if Google were to try harder, spammers would also jump on that opportunity. It's easier to mimic the patterns of a small legit SMTP server than to pose as one of a few big well established email providers with known IP ranges. They could potentially use millions of compromised PCs, each sending spam in low volumes mixed with non-spam traffic.