It is quite normal for cows to go over their due dates. I think with some lines and also some clubby lines, which are composites of different breed genetics, there is more variation. I have seen wide variations even in ET full sibs all implanted on the same day. I have had as much as 18 days between full sibs on 2 occasions, and in both occasions the first and last calves born were heifers. I was expecting huge variations in BW however, the most difference there was was 10 lb between 1st and last. That was my experience and may not be anyone else's. I am beginning to think that the cow has more to say about time of calving that we have put on her in the past. I have one cow that always fools me and calves before I am expecting her to, and she doesn't seem to even make much udder until after she calves. I have three daughters in the herd, and they do the same thing, although they are not as pronounced as their mother. All 4 of these cows seem to have very short gestation lengths. Most I have read indicates the sire has a lot to do with gestation length, and I think this is correct, however, I suspect the dam has some influence as well.
I have never induced a female that has gone over her due date, however, I do know it is a common practice. I do not worry about any of them no matter how far over they go, however it does make for many more trips to the barn or pasture. In warm weather months, I put Mother Nature in charge, and usually it works out very well.