Lack of linearity in audio levels

I send audio clips from FCP to Soundtrack Pro. I raise the amplitude and/or raise the volume in the envelope section. The dialogue, which is what I'm working on, peaks between -6 and 0dB.
However, when I return to FCP, the file is updated (because I notice/hear the changes) but the audio meters in FCP give me a different reading, which is always lower. It also sounds lower as well. Of course I can raise the volume in FCP again, but I should not have to.

Any ideas?

Best,
Rubén