I had the same idea as db4tech at first. But fortunately, he's wrong (I say "fortunately" because if he was right, this would mean that this bug could happen on all objects made of multiple shapes): the generated texture coordinates take into account the size of the whole object.
If you try with the updated models (Blue slate roof and Red slate roof) where all the slates are managed as only one shape, you'll get the same result. Maybe this problem is due the fact that he slates are inclined. I'll have to investigate further.