Because ASCOM does not expose the Offset for the camera. When using the ASCOM driver we have to abide by the ASCOM specification. And we can’t set something if it’s not exposed. Here’s the standard:
https://www.ascom-standards.org/Help/Developer/html/T_ASCOM_DeviceInterface_ICameraV2.htm
You’ll notice there is no Offset property. But Gain does exist.
The ASCOM Camera Setup dialog allows you to set other things if you choose. But SGP can’t access them.
Thanks,
Jared