The discrepancy is due to parasitic capacitance in the inductor. This capacitance shifts the resonant frequency higher, which is the cause of the reduced attenuation.

The inductor’s DC winding resistance improves the filter performance, so the experimental result should have been sharper than simulation.

The theory is correct. The experiment likely failed due to measurement error and can be ignored. The theoretical/simulated value (-50dB) is the correct value.

The simulation is flawed. Ideal component models are too complex and introduce precision / rounding errors. The experimental result (-20dB) is closer to the true ideal value.

The experimental components must have been incorrect due to tolerances, manufacturing defects, etc.

The DC winding resistance of the inductor lowers the Q factor. This highlights the importance of theory, simulation, and experimental phases; theory provides an ideal goal, simulation (when accounting for real-world effects such as can predict real behaviour, and experimentation validates the practicality of the design.

The discrepancy is due to the inductor’s winding resistance, . This resistance increases the Q factor of the circuit, making the filter too sharp for the oscilloscope to measure, which is why it appears to be -20dB.