I do not believe that reason. Yes FXCM live data is normally different because it depends on the source and has outliers, but for FXCM historical data they combine their price sources and remove outliers, so it should be always the same. Even if not, the differences should be irrelevant on bar periods above 60 minutes.

Anyway can you not save Zorro historical data in CSV format and then import it in TV? Or is import also only for premium subscription?

Or you test not Forex but stocks or ETFs, they should have no data differences.