What if the design was not for combat ?
Lets say the seax was used both as a weapon, but mainly as a tool.
And that the backside was used as a small handy "club" or "hammer" for various tasks.
Cracking nuts, pounding meat so it becomes tender, and things like that.
A point on the backside to concentrate force of such a "club" could be very handy as a place to concentrate force