I would guess you get inserted into the closest frame/spot in the update list when you log in to the arena. After all the map is updated immediately after you log in.
During scenarios I am always near the last in our squad to get a dar update. Particularly noticeable when every one is staring at a darbar waiting for it to move from one sector to another. This late update is consistent every frame. I even know which squaddie to ask because he usually updates first.
I wonder if my habit of entering the SEA early is contributing to this?
Could it be that I randomly always select an update slot ( by my login time) that is latter than the the vast majority of my fellow players.
I always assumed that it was an FE or ISP characteristic ( something unique to me)that was defining this.