Audio level changes from browser to timeline

Hi All,

I'm seeing something in FCPX 10.3.4. I've never noticed before...

I'm working with some mono voice-over audio supplied by a recording studio. I'm finding that the audio level drops by about 6 db when the clips are placed in the timeline compared to when I listen in the browser. The level meters show this also.

This only happens with mono tracks. A stereo track (such as a piece of music) is the same level in both the browser and the timeline.

Can anyone explain this? I'd like to understand what's happening.

Thanks!