I see fairly often different expert advisors being developed to trade on any pair. However, they are never developed or tested on all currency pairs. Usually there’s only one pair and it’s made and tested on that. But traders still use it on random currencies and see totally different results. I think it is only logical to have a robot made for one currency pair and trade with it on that one actual pair all of the time.
That is what Forex Brilliance authors think too and they have developed a suit of EAs that trade on specific major pairs. There is not any confusion as to what to trade it on and on which currency pair it should work better. I believe more developers should use this practice.
Not only that, when you are trading by hand you should consider that to be true for your manual system too. It is a matter of chance, when you test and modify a system on one pair, it’s likely to perform best on it. Naturally, I don’t say that there are no systems that are universal, but it is’s lots more difficult to create and run such a expert advisor.