Poor service

I agreed to a fixed rate homelone in June 2024, all of a sudden when the interest rates decreased Standard Bank decided to increase my homeloan, when I called I was told the system did a recalculation and that I paid short. This is unexceptable, what is the use of a fixed rate when they can decide anytime to change your homeloan rate? All the agent could tell me is the system did a recalculation on the rate and going foward this is the new rate. Did we not agreed on contract what the amount would be until it is up for review again. Why do they get to break a contract agreement without any fuss, we as consumers must put a stop to this as we as clients are being abused finacially.