In fact this kind of RFEs do not have anything to do with the job for which 485 is in process. Whenever they have doubt (mostly random) they want to know that you are "in-status" right now. Theoritically, they should compare your earnings with the salary mentioned in your current H1 (if you are in H1 status) LCA and not with the one mentioned in GC LC.
Now, if your LCA says 60K, green card LC says 80K, you are getting (right now) 65K (assuming prevailing wages for this postion = 55K), will they refuse to approve 485 ? Theoritically, they must not because GC job is for future. But, if you are working for the same employer, they might say that if the employer cannot give the salary as mentioned in GC LC right now, what is the guarantee the employer will pay that much after GC is approved.... this could be a ground for denial.
And by the way, it has nothing to do with "national average", or atleast INS does not care. This is the job of Labor dept. not to issue LCA or LC if the salary is below "prevailing wages" in that geographical area for this type of job (again, nothing to do with national average)
Just an openion.