Excessive Data Costs

I recently changed my contract in an attempt to reduce my exorbitant bill. I have a contract which includes 2 GB of data and I never use more than 1.5 GB. However, I'm being charged for additional data usage? I'd like to know how this is possible? I feel that Vodacom is ripping customers off with exorbitant charges. I've been a Vodacom customer for several years but considering Cell C or any other provider as an option.