From the "Model Parameters" section of "spectre -h bsim4":
484 w=5e-6 m Default channel width.
485 l=5e-6 m Default channel length.
So the model can define the default value of w and l if not specified on the instance. And there are default values of those defaults...
Since you are looking at the model parameters (rather than instance parameters) you're simply seeing the values of those defaults. You would not expect (as I've explained in previous posts) to see instance parameters affecting dumped model parameters; after all, the same model is shared amongst multiple instances, regardless of the w and l of each instance.