When writing to the calibration registers (SOC, SGC, SCOC, SCGC), the value read back from the register will not be updated until after the first conversion is complete. If you write the register and then immediately read it, it will still appear to contain the old value. The register control circuitry checks that all 24 bits were written before committing the value to avoid saving incomplete values into the calibration registers. As a result, the register contents are not updated until after a conversion is performed.