Contract voice minutes balance issue

Good day. At the beginning of each month I get 200 minutes which then expires after 60 days. In other words the minutes I received last month will expire end October and the minutes that I received this month will expire end November. When I do phone calls the minutes get subtracted from your balance. I noticed that Vodacom first subtracts the minutes from the balance that expires end November before it subtracts the minutes from the balance that expires end October. PLEASE TELL ME WHY IN THE WORLD DONT YOU SUBTRACT THE USED MINUTES FROM THE BALANCE THAT EXPIRES FIRST??? You first have to use up the minutes that expire end November before you can start using the minutes which expire end October..... Really not nice from Vodacom. It never used to work like this.