The voltage is dropping due to the 750 ohm resistor. The first R3 and R2 create a voltage divider that cuts the signal voltage in half since they are both the same value. That's what gives you the .749v on circuit B.
Now that voltage is applied across another resistor, 750 ohm, which will drop the voltage more, based on how much current the ADC is drawing. I'll let you figure out how much current the ADC is pulling by using Ohm's Law.
EDIT: Wow, that's a lot of current for an ADC. Now I think something is amiss...