Hooloovoo,
Have you tried asking your instructor this question? It would be interesting to hear his/her explanation...
But essentially, the Lycoming block in the R22 is de-rated. ie it is producing very little power for it's capacity. This means that it is hardly stressed at all. Which, in turn, leads to greatly improved reliability.
As far as I'm aware, certain Enstrom models use the same engine (ie Lycoming 320). But in order to supply the power for the increased weight of the aircraft, the engine is turbo-charged. This, in turn, leads to significantly reduced engine reliability.