Vodacom uses bundles that expires last when making calls then the new bundles

Why does Vodacom use the voice bundle that expires last when making calls, then there after the one that expires first? I have 2 voice bundles on my Smart M+ contract, there was lots of minutes left from last month that expires at 30/09/19 and then I have the bundle from this month that expires 31/10/19. When making calls it uses the bundle that expires last. I feel this is a way for Vodacom to make more money so that you run out of bundles faster and then have to buy more.