I would imagine that they have far less email volume to train any kind of spam-classifying AI on, as well as a smaller amount of (human) resources to spend on advanced spam classification. Google has a huge leg up in this particular aspect.
Another big aspect is FastMail can't use non-spam emails for training data. And users can opt out of having even their spam emails used for classification. FastMail doesn't co-opt people's data for their own use so they rely entirely on voluntary sharing.
