PMS5003 accuracy issue on recently purchased Airgradient monitors

I recently bought 3 Airgradient monitors side to side on a table, 2 of them give about the same PM 2.5 readings, the other one gives numbers on average 20% lower.
All are on the latest firmware 3.0.9

Monitor #1 (indoor): CreativeShot Garage PM 2.5 48 hours average 2.9 µg/m3
Monitor #2 (indoor): Bedroom PM 2.5 48 hours average 4.8 µg/m3
Monitor #3 (outdoor): Petie Way PM 2.5 48 hours average 4.4 µg/m3

I thought the Plantower PMS5003 PM Sensor was supposed to be more accurate than that? 20% or more variations seems a lot !

These readings are fully within the specifications of the Plantower sensors which are:

Accuracy: ±10%@100~500μ g/m³, ±10μg/m³@0~100μ g/m³

Source: Indoor Air Quality Monitor

In general particle measurements in lower concentrations have a higher error because there are just not so many particles to detect. Even reference instruments sometimes disagree by 20-30%.