I have two Japanese forging hammers, neither of which is over 15 years old. They have rectangular eyes, but the hafts are shaped oval except where they enter the eyes. I got out my bevel gauge, and measured the head to haft angle, and it was 88 degrees on both. One hammer is 3.5 pounds and the original haft is 12" long. The other hammer is 1.75 pounds and the haft is 12 1/4" long. It looks like both heads were started with square stock, as the polls and eye-cheek areas are squarish, with the polls having chamfered corners. The heads are shaped rounding with circular faces. When you pick up the hammer, there is no mistaking which end is going to hit the hot iron. I have drawn and measured other Japanese hammers, and the proportion of poll end to eye center compared with face to eye center will vary.
As to the whys and wherefores, I sometimes think that the hammer styles from a particular country are a matter of happenstance. Perhaps a guy was a super smith doing wonderful work, and others saw his hammer. "Hey, I'm going to make a hammer like that."
Sayings and Cornpone
"Iron in its happiest state is rust."
Jim Gobling, expert on old English anchors