Forex is over-the-counter market; therefore, there're no global consistent rates like those of stock market. Not only forex prices vary from broker to broker, but also volume. In metatrader, volume is computed by counting ticks for the bar. That is if EURUSD changes 1000 times in 1 hour, you'll see 1000-point volume for the last 1H bar.
I guess trading systems must be exclusively tailored to work in forex. Some indicators like Force Index that utilize volume variable doesn't work consistently.
Ok, brokers normally provide the same data feed for both demo or real accounts. But it doesn't mean that system that works on demo will work in real account. It's a whole lot of inconsistencies in forex ranging from rates, volumes, and order executions.