Multisim treats digital input and output as high or low behind the scenes, Multisim converts a high to the component default voltage level which in this case is 5V. By assuming a high is 5V, Multisim can simulate faster.
If you want the CMOS components to use the VDD voltage:
- From the main menu select Simulate»Mixed Mode Simulation settings
- Select Use Real pin model.
This setting will force Multisim to add a pin driver to every digital input and output to convert a high and low logic to VDD and VSS.
This setting will also cause the simulator to take longer than usual to finish depending on the circuit complexity.