Different Indicator Values on different brokers?

I noticed that when using indicators for back testing (e.g. cci overbought/sold) that results are completely different between brokers. What may seem like a good setup on one, is not on another. E.g. One broker shows the cci overbought but the other is not yet overbought. Is this normal? Why are the values not the same if it is the same currency pair/timeframe? I assume it is to do with the slightly different quotes that make the bar data that then calculates the indicator values. How would it therefore be possible to test a strategy using indicators accurately if the indicator values are different on each broker?