The reason we request two ss is because there are many bugs in bf2 that can look like hacks that aren't. For your case, reduced fog, there is a bug where if you were commander the round before and were typing in chat at the end of the round, you will spawn the next round and have no fog at all. There are other similar bugs in bf2 for fog and hence the reason we require two for reduced fog screenshots.
The other most common rejection for not having two ss is for esp (in particular names showing behind walls). Many people submit screenshots with one or two names visible behind a wall. If an enemy is visible, gets in the players line of sight, then ducks behind a wall again, their name will show up red for a brief amount of time afterwards. Because of this, we require two screenshots.
So all in all, its to make sure the screenshots are not caused by bugs. Just our method of quality control.