mpak - You are apparently confused about how a dyno works. Dyno results are subject to many variables, including air temperature, atmospheric pressure, physical differences between engines, dyno operating procedures, etc., just to name a few. The same bike could easily show a 2 HP difference between two runs on the same dyno; comparing runs of different bikes on different dynos is meaningless. If you had wanted to know the effect of changing your exhaust system, you should have made a dyno run with the stock exhaust, installed the new one, and dyno'ed the bike again, preferably on the same day. The only real value of a dyno is to make such before and after comparisons on the same dyno and bike.
With that said, an engine is basically an air pump. As Haggis noted, removing restriction from the exhaust (i.e. pump output) side will only increase power if the engine was being prevented from making maximum power due to exhaust restriction. On some specific bikes, that is undoubtedly the case, due to differences in their individual engines (e.g. tolerance stacking, etc.). In other words, if you dyno'ed a lot of bikes with and without a given exhaust, some would show a significant increase in power (e.g. typically 4 - 6 HP peak) others would not. Reducing the intake restriction as Haggis suggested may increase the amount of power your engine makes, but only if it is capable of using more air. Other modifications may be required for it to do so (e.g. porting the head, installing higher performance cams, etc.).
Decades ago, factory exhausts were typically built as inexpensively as possible and replacing them with a more carefully designed and built exhaust system would almost always produce measurable power improvements. However, modern OEM exhausts are very well designed and built and such gains are no longer assured. Moreover, the loss of some low-end torque is a natural consequence of reducing exhaust restriction (i.e. it "proves" the new exhaust is functioning as intended). These are all well known facts that you could have found by reading this or almost any other performance forum. Doing so might have saved you 1300 euros or you could just wait for a clear, cold day and dyno your bike again.