Fixed Contract Amounts Fluctuate

Good Morning, I would like to find out how a signed contract with fixed amounts can fluctuate and be different every month. Not one month goes by and the amount on my contract for all 4 numbers are ever the same. I thought signed a contract with a fixed amount with the deal that was taken out the amount should stay the same, considering i did sign a 24 month contract. This has been going on for sometime now and i just cant handle this anymore. Just as a person thinks all is good with Vodacom something else comes up. Please have a look into this as i will not be paying amounts that i should not have to be.