Why are the two sides of Fuji np-w 126s battery concave?

Don't believe that depression is to prevent inflation in plateau areas or during transportation. In fact, it is a counterfeit product with high imitation. Recently, I bought an x-t3 in Hong Kong, and also found that the battery and charger have been replaced with high imitation ones. My previous battery, w 126, never sagged or expanded at high altitude!