Editor playback audio differs from final exported AIFF mixdown. Help!

Hopefully someone can help me with a problem that is causing many a hair to be pulled out in frustration. Only recently I have noticed that my final exported AIFF mixdown file sounds ever so slightly different from the playback I hear in the editor. My workflow is as follows:

• Create new multitrack session (44100 hz, 16-bit),

• Once mix is finished, Export mixdown (AIFF, 44100 hz, 16-bit),

• Create new "mastering" multitrack session (44100 hz, 16-bit),

• Import AIFF mixdown file to session editor, apply desired effects (eg "Mastering", "Parametric EQ" etc),

• Once happy with mastering, Export mixdown (AIFF, 44100 hz, 16-bit),

- This is now where I do anA/B test, and notice the inconsistencies -

• Create new track in the multitrack "mastering" session editor, and change the "Output" from "Master" to "Default" (ie bypassing Master),

• Import mastered mixdown AIFF to this track,

• Compare this exported track to the original in-editor version by solo-ing back and forth. I discover that the editor version of the track seems to have more range/it just sounds that bit better/bigger/not as 'dull' in comparison to the mixdown file.

I have two fundamental questions:

1. What is causing this discrepancy between the quality of in-editor playback, and the quality of the final exported mixdown AIFF.

2. How can I make sure the quality of the in-editor pkayback exactly matches the final exported mixdown AIFF?

Thank you in advance. This is driving me nuts!