Contract payment

I have had a data contract with Vodacom for about the last 8 years. I cancelled the contract and was told it would run till the end of June - on the 25th of June my data contract was cancelled, leaving me with 5 days without any data for something that a months payment was taken for, this is a 6th of a month yet I was not billed for 25 days but a full 30 days, I have spoken to the call center as well as gone into Vodacom Gateway and no one can advise as to why this happened, this is the second time Vodacom has done this to me, when I changed my phone contract the deleted almost 3000 minutes I had saved and paid for and again when I tried to get them back no one would budge. I just want to know how you can bill some one for 30 days but only offer a 25 day service??????