I had one of those once, i think it was an older craftsman, it probably got through the production process as a mistake. An oddity, but not common i think.
If you check our various ignition wrench sets you will find it common to have different ends on the wrenches. That way if your dealing with a nut and bolt you have two ofthe same size wrench openings in one set. You do not have to have two sets of wrenches if you need two of the same size. Spend a bit of time in the various catalogs and you find your style- box/open also 15/30 degree open ends. Handy if you need them.