Readout noise.
The IMX294C sensor has HGC and LGC mode. HGC mode can reduce the readout noise a lot. In the whole gain range , if working under the HGC mode, the gain is 1.6e to 1.0e . If working under LGC mode , the gain is 6.8e to 4.2e.
Under lowest gain of the HGC mode, the fullwell is about 14.5ke.