Fps can vary for many reasons. It can depend upon the number of people who are in the same outpost or instance as you (but not specifically the total number of people online). It can also change because of certain graphics elements that require more GPU power in the particular instance you're in and/or the number of graphic elements to render, etc.
Your ping can also affect the overall smoothness of play.
Everybody has this 'problem', but as I said, if you have enough GPU/CPU power, you don't notice it unless you have vsync off and specifically observe your fps.
Last edited by Quaker; Apr 23, 2012 at 01:32 PM // 13:32..