This is a standard NEMA 1-15P plug as used in North America. One blade is large if the plug is polarized, but you rarely see polarized equipment nowadays so it's become rarer to see the polarized plug.
As for the blade spacing, there's a fair amount of play in American plugs and sockets. The spacing can be off by a bit without it really mattering, and the blades bend fairly easily on top of that so it's possible your measuring implement (calipers, perhaps?) squeezed them closer together while you used it. On top of that, there's additional imprecision added by the use of that hinge mechanism.
It also comes to mind that the spacing depends on how you measure it; the blades are about 1.6mm thick, so if you measure inside-to-inside you would expect to measure about 11mm, rather than 12.7 (which I believe is the specified center-to-center distance).
A physically identical plug is also used in Japan, though I don't know as much about that.