NEC requires the cabling & OCP to be sized 125% but immediately grants an exception if the breaker is rated for 100%. My understanding is that this is due to breaker construction and heat tolerance and the tendency of simple thermal trip units to nuisance trip. If you look at the actual trip curves for molded case breakers, you'll see it's not a clean line, but a wide shaded area where the breaker may trip at different times in different conditions (ambient temp, how hot the breakers above/below it are, manufacturing tolerances, etc). Heat soak can shift the thermal trip portion of these curves well to the left. You can safely run an 80% rated breaker to 100% with non-continuous load, but if you keep them at 100% too long, the thermal trip unit could trip below the OCP rating.
Typical trip curves:
http://static.schneider-electric.us...%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdfVirtually all the inexpensive molded case breakers I see for residential and commercial use are 80% rated, but once you get into the big industrial breakers with adjustable electronic trip units, they're all 100% rated. I don't think I've ever seen a 100% rated 20A breaker, but I also don't think I've ever seen a 1200A+ breaker that wasn't 100% rated. Which makes sense: the small breakers rely on internal heat buildup to tell if it's overloaded, but the high-end trip units are using CTs and actual current measurements. The same physics applies to overseas breakers whether the local codes require it or not- it's not like there's a difference; they all use the same thermal trip principals. Actually, perfect example: Schneider will take the same breaker and slap an MGE or SquareD label on it depending where it's being sold, but it's literally identical.
