So sorry, Me so stupid! I was under the impression that you were using the word 'belly' to refer to the curving 'smile' of the blade.

Although it doesn't have the same form, the ground-in thickened 'belly' of the blade serves the same purpose as a T-form structural reinforcement. It stiffens the blade without significantly adding to its weight. Actually, both blades have the 'belly', except that on the one it is very pronounced and the on the other it is a mere ground-in thickening.