I'd also say that they wouldn't be angled in towards each other, my 531c steel roadie with similar lugs are parallel. If the correct size of post is fitted, when the bolt is tightened the gap should not close up very much, that it has is an indication, as has been said, that bolt has needed to be tightened too much to grip post. The less movement to lugs means bolt isn't put under a large bending strain, and consequently breaking. I'll change my mind again to now agree that the wrong size post has been fitted from new.