Yep, that is what Colt marked them. Without a time machine, I doubt we will ever know why. Note that a .455 magazine is wider than a .45 ACP magazine and won't fit in the .45 ACP pistol.
At a gun show a couple of years ago, I mentioned that to a dealer who had a .455 magazine on his table. He called me "a dumb sh*t" and said they were the same. He picked up a .45 ACP pistol and tried to put the .455 magazine in it. It wouldn't go. Then he hammered the mag on the table, then on the floor, bending the lanyard loop and the base plate lip. He finally got it almost in, turned to me (I was standing there, horror struck) and said, triumphantly, "See, dummy, it fits." I walked away shaking my head at a fool who would ruin a $150 magazine because he was too stupid to concede a point.