What would be causing the sensor’s relative humidity readings to be much less than an accurate reference’s readings?
The most common source for a low RH reading is excess heat occurring near the sensor, such as PCB heat. The sensor and surrounding air must track the ambient temperature in order to accurately measure humidity. In general, every 1C of heat decreases the RH reading by 5%. A more detailed relationship between temperature changes and RH readings can be found in Appendix B of AN607. Please reference AN607 for design considerations such as sensor placement and PCB layout.