well, as far as I know (correct me if im wrong) "arma2" on there will include all searches with arma2 in them, that includes arma2 addons, arma2 editing, arma2 moding, arma2 servers, anything with arma2 in it will be included. And unless people have recently started switching from google to other search engines, I dont think it would make a difference in this kind of result. As you can see by comparing it with ofp, there is definitely something different. There were 2 main spikes, first when Arma 2 was released, and second when OA was released. but you can see they quickly fell back down. I believe this means that even though alot of people get the game, Most of them are not staying with it. They play the multiplayer until they get bored then stop.
If people were editing, you would (as with ofp) see a far slower decline because people will constantly search for new addons, new missions, new scripts, new islands, and this would prop up search volume.
you also have to remember that there is no data for the beginning of ofp, the best ofp years were 2001 to 2004. I'm guessing if we could see the trend it would be way up during that time. Even without the ofp 01-04 data, you can plainly see that arma2 after only 1.5 years is already to the point where ofp was after 5 years.