Why our Air Particle Counter with both quantity and weight for PM2.5

Our particle counter use the imported laser sensor to check the result.
Acutally the particulate unit is μm. and it is quantity to account.
But the world account way always use weight because it is easy to confirm with WHO request.
The WHO request for the PM2.5 is 10μg/m³, this is under the weight request.
So we have two way in the counter both the particulates quantity and the weight.