The PM2.5 sensor on a purifier is often placed close to the vent on the purifier, which is the cleanest part of the room, and people cannot breathe from there. The air outside the purifier must be worse, and will vary across the room. To get an accurate representation of how the air is diffused across your space, you must measure from further away from the purifier.
The PM2.5 sensor on a purifier is a bonus feature, so the quality can be mediocre. The reading may be inaccurate, and the sensor may not respond swiftly to air quality changes. As a result, the sensor cannot tell you the exact air trend in the room.