Chronographs can wander off (usually up), especially the little hand helds.
At scenarios, one will wander off and that ref becomes a punch-nazi because he thinks everyone is shooting over. Worse is at evening turn-down when the paint swells up and people fresh filled their CO2 tanks, so they tend to shoot hot anyway.
It's always best if you can cross check with another chrono. The big reds don't seem to fail high as much as they can get lost in reflections at high temperatures (95+F)
The yellow ones show +/- 2%, which should be common. That's +/- 6 fps just for normal operation. Handheld Radar Chronograph for Paintball Gun