I thought of another explanation - when a keris sheath is made for a keris, a well-made sheath's only point of contact with the keris shd be the ganja. Hence, the blade is 'suspended' in the sheath, supported only by the ganja. With time, and with all the wearing of the keris (worn upright), the weight of the blade causes the ganja to warp away from the main blade.
A similar case would be with katanas/wakizashis/tantos - the blade suspends in the sheath, supported by the habaki and the mouth of the scabbard,
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