To the OP...can you give more details as to the wheel style or number so that we know what specs we're discussing? Also, double check to see if the wrong tire was mounted on to the wrong wheel. If the wheels are staggered...and the fronts have a lower offset...perhaps the "315" tires were incorrectly mounted onto the front wheels...which would "push" the tires out further than the wheels with a higher offset. Just a thought.
In regards to the X6 having different size center bores (front/rear)...you can see the chart below that it is the
e71 X6 and
e70 X5M that have the split axle center bore difference: