I came here for the exact same reason. Until I hear otherwise, I was going to go with their WIL being +2 as that matches the descriptive text imo
Viewing post in Hansen Expansion (v. 2.2) comments
Hey! Yes the voles do have penalties on their stats because they get stronger Boons. Having them be at a disadvantage with rolls to have better boons for their team was intentional.
Also, the alternate player species are supposed to be a challenge compared to playing as a mouse. They’re not to stand in as races like in D&D, where they have comparable pros and cons. ALL alternate player species are more difficult than playing as a mouse. The vole with its negative stats, and the frog & lizard in that they can’t normally communicate with rodents- you’ll need a book to do so.
Hope this helps.