Have a look here for some indications -
http://www.dynamicflight.com/aerodynamics/flapping/
There of course could be other reasons!
The EC120 is made by some French people and some Chinese people and also some Singaporean people and no doubt some German people and also in the near future even Australians.
The Agusta 109 E (Power) has at least 15 degrees difference in incidence on the horizontal stabiliser from one side to the other. No need for Gurneys because the design allowed it.
Stand behind a Bell LongRanger and look at the different angles on the vertical fin and the endplates. They oppose each other.
Check out the little tab on the top at the front of the horizontal on a ShortRanger. Ray Prouty explains that this makes the stall of the stabiliser less noticeable.
The rear gear fairings on the 120 are also not to make it look nice.
The AS350/355 family have all manner of variations of flaps depending on the model in both horizontal and vertical. The 355N has about 300mm tacked onto each end.
Try and find the "Prouty" books and all will be a lot clearer and will "possibly" explain the methods. Aerodynamics around the rear of a helicopter are hugely complex and even Mr Prouty admits that a lot of it cannot be clearly explained as to the true reasons. The dangling bits just fix it.
I have to contend with a Vne reduction to 140 knots at the moment due to cracking of the tail rotor blades. I am told it is from the turbulence generated by the mast fairing etc.etc. Trouble is the aircraft will fly faster than this S & L. Owner is not well pleased. Only fix appears to be complete re-design of the blades. How long - who knows.