Balances in my account not adding up

I have seen for a long time now that when ever I make a purchase from my account the balance that shows on my sms is not the same as i worked it out. Eg if my balance in my account shows R100 and I purchase something for R50, my balance suppose to be R50 but no this is not so. My account shows a balance of R30 rands. It is far less than it should be. Where is my money going too. This is plain *******. I would like an explanation for this.