Price differences

I tried booking a flight from Cape Town to Lanseria for 21 February 2015 @ 16.45 and tried paying with a credit card but it wouldn't go through although I had more than sufficient money in my account. I then called the call centre to do the booking but the price I saw on the internet was R869 and the price the agent quoted me at the call centre was R1095..this is more than R200 than what I should have paid. The agent was rude and could not provide me with a reason for the difference. He just said that is the price that he is seeing on his side. How can these prices differ so drastically?