AD7793

Hellow.

I'm mesure 100 mV from Pt100. I set gane from 1 to 128 and using internal Vref.

I recived such 16 bit digital codes from ADC:

x1 - 5413

x2 - 10819

x4 - 11038

x8 - 21722

x16 - 25780

x32 - 26957

x64 - 28497

x128 - 56952

I do not understand why each successive value of the code is not equal to the previous multiplied by 2?

HELP PLS