Because there is a difference between filtration class and vacuum class.
The filters might be good enough to be an ' M class ' filtration ( because they are HEPA filters I believe )
But for a vacuum to be rated as m class u need more then just a good filter. Most important extra is that your vacuum continually checks the flow rate of the air. If it drops under a certain level ( indicating something is clogged up and your vacuum isn't working as it should ) then the vacuum needs to give you a visual or audible alarm.
There's a few other little things your vac needs to do/ have to be considered an M class but the reason above is the more important