You never really know with the USPS. It's possible that the Air Mail stickers weren't noticed, or the package was improperly stickered, it's also possible that it was marked, but not paid for, it's also just as possible that Japanese or US Customs held it up for awhile due to incomplete/improper documentation, or just because they wanted to.
One of my many positions at the unnamed computer engineering firm I worked for was heading their shipping dept. for a little over a year.
I don't think I ever managed to get anything to Japan or China in under 30 days regardless of how I sent it if it went out via the USPS.
