It was definitely a net total when I got the accomplishment, they may have changed it though. The first time you get to 1250 total is when you get the 500. If you get 1249 then drop down to 1100 you have to get 150 more before you get it. That's how it used to be.
Gotcha, that would make sense. My current net Trophy count is 988, however, when I click on the lady to see my achievements I have 1047/1250 on the multiplayer battle line. When I have won recent battles though the 1047 hasn't been changing, so I guess maybe it is waiting for my current 988 to exceed it. I thought the 1047/1250 would have said 988/1250.