That would be for a expert to answer, but my huskys have 2 filters too. One mesh like that green stihl HD for winter and flocked one for other times that catches the fine stuff. If I run my winter filter on the husky it will let dust through just like the green 1653 stihl one in good weather.
Just a guess, but winter time moisture might have something to do with it. I am really clueless to why. But that green HD winter filter is a no no in my book unless you want to ruin your saw. JMO and it dont count for much.
It's all in the Stihl IPL's if you want to look for yourselves. I would get the first one on this stihl IPL list.
0000 120 1654 Air filter HD
0000 120 1653 Air filter, wire mesh THIS IS THE GREEN ONE
0000 120 1601 Air filter, fleece AUS