Sorry, I forgot the second part of the thread.
At the early days of WW1 the (oldfashioned) Prussians officers responsible for the new branch of army aviation considered it to be nearly impossible to bring down a plane with the help of another flying machine.
Flying itself was quite dangerous at this time (even without being shot at or trying to attack another plane). So they thought 5 kills would be worth the PLM.
In my opinion it was easier to have a confirmed kill in 1915/16 instead of 1918. The Fokker monoplane had no counterpart and there were no specialised tactics (the ability and knowlegde of the pilot was the most essential thing).
(I´m quite sure that there will be other opinions.) *
Best regards
Volker Nemsch