Due for upgrade is a ****

I have upgraded my contract in 2021 December. I recently ask for confirmation from Vodacom when is my contract ending and they said 2024 April. My question is why do I have to pay 28 months for 24 months contract. No one gave me an explanation.

Instead they said it’s because I took my contract before the other contract end. When I took the contract the sales person told that I will have to pay 2 contracts until April 2022 which I did pay and I don’t own anything from my previous contract.

I started paying my contract in January 2022. Why do I have to pay extra 4 months?