Some bottles have different size lips, and some cappers have different size crowns. While they look the same they are going to be just slightly different. What you try to cap the bottle, the neck will shatter.
26mm is the midsize cap, which is common throughout the world, but there are larger 29mm size cap, which is used in Europe (common on Belgian beers) and on champagne bottles. Some breweries in the US are also starting to use the 29mm caps.
I have not found an outlet for 29mm caps, but I also didnt look that hard. When I found out my capping crown was 26mm (which you will get from a homebrew shop) I just started only collecting those 26mm bottles.
Hope that helps!