Low framerate on certain spots

On Golmud Railway I have like a 90-100 fps framerate most of the time. On other maps its like 80-90 and Paracel Storm is like 70-90.

But is it normal that the framerate goes down to like 58 fps on specific location on a GTX 780? On Siege there are several spots where my framerate goes down to like 57/58 fps. Almost every map has those spots and I have no idea why the framerate goes down since there is nothing special on these locations/spots.

On Golmus Railway I can walk through one of the small villages while there is stuff exploding left and right and still have a +80 framerate, but in Lancang Dam or Zavod 311 I already get a 70 framerate from just facing the two buildings in the middle of the map.

I also noticed this:

I played Goldum Railway and was in one of those small villages that I mentioned. I was on the roof of one of those buidlings and looked down to the road and the other buidlings and had 60 fps which is very low compared to the usual framerate.

Later I played the same map again and randomly was at the same spot and tested it again. So I looked down and there was stuff exploding and so on, but my framerate was steady 80 fps and barely made those jumps from earlier. ( same mode and so on, also the buidlings were damaged too )

I had this a few times that in one game the framerate was noticeable lower than in another while it was always the same mode and so on.

This might sound confusing, but I dont know how else I should explain it.

Any help?