LC Power filter not achieving modelled attenuation
I suspect that your scope probing is not ideal. Did you use this type of probing measurement to reduce to &quot;loop&quot; pick-up formed by a conventional scope's probe: -


You can do a simple test to prove it is your probe technique - connect probe top to where the earth clip connects and see what is picked-up - ideally there should be no signal picked up because probe tip = grounding point. I'll guess that you pick-up quite a bit of noise and spikes if you try this.
SMPS's are notorious for screwing with a badly connected probe and you probably need to use a ground spring: -

See also this article on the same subject entitled Power Tip #6: Accurately Measuring Power Supply Ripple.