Someone posted a Medium article [0] on the /r/bayarea subreddit where they did some digging into this. Apparently the sensors used by Purple Air rely on a constant that represents the average density of the particles it detects. Because wood smoke particles are less dense than typical PM 2.5 particles, the resulting AQI values are too high.

The Lane Regional Air Protection Agency has developed a conversion formula that is built into Purple Air (if you apply the LRAPA conversion factor in the UI), so you can have a better comparison.

[0] https://medium.com/@16fcali/understanding-purpleair-vs-airno...