So. I downloaded a manual and found this:
"All four tires must be the same in terms of manufacturer, brand(tread pattern), construction, de-gree of wear, speed symbol, load index and size. Mixing tires of different types, sizes or degrees of wear can result in damage to the vehicle’s power train. Use of different types or sizes of tire scan also dangerously reduce controllability and braking per-formance and can lead to an accident."
Mmm. I guess I can somehow measure the tires to prove that they are the same size, or is Subaru just being cautious.