Here's the test I have just done.
I created mono 1k tone in Audition at -18dbfs
Imported into Premiere
Created a new sequence with a MONO master track and MONO sequence tracks (it does not work with standard tracks, this is where your level difference might be coming from))
Added the tone to a track
It played at -18 both on the track meters and the output meter
Exported as a Mono wav, reimported into Premiere
This too plays as -18