A couple of thoughts, & I may just be on the wrong track:
1: Is your windows media player using the same outputs as Cubase? Mine isn’t so the volume isn’t like for like.
2: A way to test it -
Often you may have inserts on your stereo out track/channel, such as a limiter for the final mix. It’s easy to forget, when you import back into Cubase, to bypass any master out effects.
Create an additional out - name it stereo out 2 or final mix.
During the export, tick the boxes saying import into pool & project upon export.
After export set your finished track to Stereo out 2, perhaps with Voxengo as an insert. Mute stereo out 1 and do a side by side comparison.
Then mute stereo out 2 & compare again.
You can also right click on the final mix track & get the statistics.
Just a couple of ideas…
Neil B