Tonight I opened a new (to me) brand of wine (Geno Auriemma Chianti to be exact, and not bad!) and when the cork broke on opening, I attempted to insert an old cork I had around and found it didn't fit. Upon closer measurement it was about 1mm smaller in diameter, as was the bottleneck inner diameter when compared to another brand.
Up to now, I had always assumed all bottlenecks (and accompanying corks) were equal. I guess this is one area the standard setters haven't gotten around to.