Fixed interest rate on home loan

In October I called standard bank to fix my home loan interest rate. Everything was completed. In December I got a message saying that my home loan interest rate has changed and I will now be charged a different amount. Called in and they never got back to me. January same thing happened. They then advised every month will be different based on the days in the month. So February will be less as it's less days. Only to find now in February I will be paying more. What the heck. Fixed interest rate is clearly not fixed. The system is on a fixed interest rate but every month is like a gamble. What's the point of having it fixed then. This is becoming ridiculous. Now I have to call in again. Only for them to tell me it's fixed on the system but they not sure why I'm being charged a different amount EVERY SINGLE MONTH. VERY BAD CUSTOMER SERVICE. <br>