German cars are designed for 95 octane, and it would not surprise me if Stihl followed the same line of thinking. In the case of cars it is somewhat rational: the additional energy used in producing the higher octane fuel can be recouped in the better fuel economy it delivers if you have developed the car's engine to require the higher octane. This mostly means you can raise the compression ratio to the limits allowed by 95 octane instead of 91, and the higher compression ratio improves your fuel economy. When you trade off the higher fuel economy achieved in the car, against the additional energy consumed in the oil refinery (or the additional octane improvers added to the fuel), you can at least break even if everyone does their part correctly. The practical difficulty is that outside Europe, most fuel is 91 octane, and if you detune a 95 octane car for 91 octane fuel without reducing the compression ratio, the fuel economy penalty is fairly severe (you have to do it by retarding the ignition).
So, my guess would be that if Stihl recommends 95 octane (and I doubt they do) it would be a sort of German cultural thing, not for any more sensible reason. Globally, most of their customers would be using 91 octane fuel, and they would be most unwise to design the engines for 95 octane. However from what ABC1 said, it sounds as if the recommendation came from a local service mechanic, not Stihl. The mechanic may have been concerned about the possibility that 91 octane fuel might contain ethanol, as Joe said. If that was not the reason, my guess would be that he/she simply passed on some kind of street thinking that was not based on reality.
