Sensor's RH readings at high humidity drift over time. After exposure once the humid environment drops to a lower RH, there is a shift in the sensor's accuracy.
Prolonged exposure to high humidity will cause gradual drift of the humidity sensor. All Si70xx sensors have on-chip heaters that can be used to counter local high humidity, help prevent condensation, and reduce the drift. Please refer to AN607, pg.28, for a more detailed explanation on how to estimate RH with heating. For example, turning on the heater with control setting 0x3 heats the sensor about 5C (depending upon system design) which results in approximately 23% drop in the local RH reading.