5.1 Surround sound stems set up and audio Level Changes.

I am running a test to see what FCPX does to some 5.1 stems that I have imported and then will export as an embedded 6 channel Quicktime.

I have a Project (Timeline) With 6 surround sound Channels.

The Project is set to Surround. Each Audio Channel is set to:
Volume and pan 0
Pan Mode 0
Channel Configuration MONO.

I goto share and Export as a self contained quicktime with 6 channels of embedded audio
BUT the audio levels of the embedded stems are different to the original stems...Why is this?

Ultimately what I am hoping to do is work with six stems from a feature film which I can edit a sequence to and then export the surround sound so I can encode it for further use.

Suggestions Please.