As Tar Shot is properly considered a type weakness, and not a generic damage multiplier in the mold of Fluffy, the game is considering that to be a 16x type multiplier internally, whereas Bug/Steel + Forest's Curse + Fluffy would only be an 8x type multiplier and an extraneous 2x factor elsewhere. Of course, you could also replace Bug/Steel with Bug/Ice (Snom) or Ice/Steel (Alola Sandslash).
The only other way to get to 16 is by having an inverse battle (something that's never been playable in Sword/Shield, but has in earlier games) and using Flying Press after your ability has been swapped to Normalize. In fact, by hitting an Aggron under Trick-or-Treat with those conditions, you can uniquely bump the type multiplier all the way up to 32.