Error in simulation results for balanced pi type attenuator
Try adding the 500 ohm source resistance and 500 ohm load resistance...
If you remove the three attenuator resistors, you'll get 6 dB attenuation from V1-to-load resistor R3.
If you insert the three attenuator PI-resistors, you'll get 26 dB attenuation from V1-to-load resistor R3.

Normally, V1 is not accessible as a bare voltage source...its source resistance of 500 ohms (R4) is part of the source. You would measure attenuator input across R1 (below). Then the attenuator output across R3 (below) would be -20 dB.






simulate this circuit &ndash; Schematic created using CircuitLab