We use the Sensirion sensors in our Pulse One. They are some of the most accurate sensors available. They've got great accuracy across a wide range of temperatures and humidity values, and have good hysteresis performance.
In addition, different sensor technologies can have different performance characteristics (hysteresis, accuracy over different humidity & temperature ranges, etc). These factors can lead to different readings. That's why sometimes sensors can be reading the same, but as your environment fluctuates, the readings diverge.
fig 1: hysterysis
Differences from Other Sensors
Imagine you have 3 clocks. Two of them are cheap clocks, and one of them is an atomic clock. Do you trust the two wrong clocks? Or the atomic clock?
As long as you followed the placement guide, and have good airflow you can be assured that the Pulse is probably telling you the correct readings, and the other stuff is wrong.
If you're the lucky owner of a scientific grade hygrometer and are extremely confident that the Pulse is wrong, you can always do a calibration. If that doesn't address things, there have been some instances where sensors get damaged, and we can gladly offer a replacement. Just contact firstname.lastname@example.org and we'll sort things out.