Swampass, I'm wondering if its just your type of tire, because I've been playing around with the numbers and it shouldn't happen with most tires. I noticed your tire is an A/S, with a pretty wide tread block and also pretty square with regard to the sidewall. Compared with, say, a Bridgestone Pozenza 01, you've got more tread on the outside edges (perhaps for A/S performance??)
It is so close, that perhaps tire choice plays a part here aside from tire size alone. I also wondered what would happen if you took the hubs of those wheels and had 3 to 4 mm shaved from the hub on the inside, effectively increasing the offset.