Charting problem

Hi all, I'm quite a newbie in this wild market...
I'm testing a system with some rules...
This system using RSI and the rule is we buy/sell when RSI break a certain level.
I am using few free charts provided by FXCM, OANDA, and NETDANIA at the same time,
but I have a problem, each chart has a different RSI value at the same time, netdania chart breaks my RSI rule few times, FXCM breaks less, while OANDA's chart did not break at all...
It confuses me... is this happens to all of you? which chart is reliable for you?

All the charts are correct.
There's a difference because each chart, has a slight different quote at each tick, 5 min, 1h etc...
And if you use e.g RSI(14), which means the RSI will use the values of 14 bars, you will have a slight different RSI on each chart.