Charge for 10 meg line but supply 6 meg at best

I entered into a 24 month contract with Telkom to supply me with a 10 Meg uncapped line. The downloads at sometimes is so slow that when you watch a movie it will constantly buffer. There were times that the line speed will be 1 Meg at its best. The last few days it is up to 6 Meg at its best. I phoned Telkom and they told me that if you have a 10 Meg line it is never guaranteed that it will be a 10 Meg. Now my question is why should pay for a service I don’t receive, another state entity just ripping people off.