Unbalanced pi- attenuator - Problem with simulation
The attenuator design process assumes it is driven by a voltage source in series with the design impedance and terminated in the design impedance.  In your case, the design impedance is 75 ohms.  Although you have properly loaded the attenuator with a 75 ohm resistor, you are driving it with a 1 volt source across the input 75 ohm resistor.  This effectively takes the resistor out of the circuit.  The 1 volt source should be in series with the 75 ohm resistor to properly simulate the design conditions.