The issue is: does the sensor need to be turned on or otherwise activated in some way? This might include turning on or activating whatever electronics are accepting the signal behind the scenes. This is almost certainly an infinitesimal amount of time, but film simply does not have this issue. Chemicals react to photons as soon as the photons arrive. Yeah, it may take a few femtoseconds for something to start happening, but delaying the incidence of light would also delay that delay. On the other hand, if the data bus behind the sensor simply isn't on for that period of time, for example, the sensor is essentially unreactive and the photons that hit it during that time were irrelevant.