heres my theory to it
1 disney resets servers at random when there looted most servers have 5 famed and 1 legendary and several rare/coomon weapeaons im not sure about full servers the chances might be boosted but i have looted quite servers on end that im am sure have reset you can tell by lag and lot of loot drops and i have NEVER and i mean NEVER got more then 2 famed in 1 server in a whole day of playing so i recommend after getting a famed switch to another server with friends the amount of ppl at the enemy also help your chances like someone said in a earlier post 5 or 6 ppl with you can greatly boost the chances of a skull at a boss also the KPM ( kills per minute ) also matter the faster u kill the more loot drops the better the chance of getting a famed after the server gets looted it will dtop famed/legend
Hope this helped
