This is example of statistic embedded into gpx:
- <gpxtrkx:TrackStatsExtension>
<gpxtrkx:Distance>9250</gpxtrkx:Distance>
<gpxtrkx:TimerTime>19281</gpxtrkx:TimerTime>
<gpxtrkx:TotalElapsedTime>34832</gpxtrkx:TotalElapsedTime>
<gpxtrkx:MovingTime>10982</gpxtrkx:MovingTime>
<gpxtrkx:StoppedTime>8319</gpxtrkx:StoppedTime>
<gpxtrkx:MovingSpeed>1</gpxtrkx:MovingSpeed>
<gpxtrkx:MaxSpeed>3</gpxtrkx:MaxSpeed>
<gpxtrkx:MaxElevation>1239</gpxtrkx:MaxElevation>
<gpxtrkx:MinElevation>384</gpxtrkx:MinElevation>
<gpxtrkx:Ascent>435</gpxtrkx:Ascent>
<gpxtrkx:Descent>624</gpxtrkx:Descent>
<gpxtrkx:AvgAscentRate>1</gpxtrkx:AvgAscentRate>
<gpxtrkx:MaxAscentRate>0</gpxtrkx:MaxAscentRate>
<gpxtrkx:AvgDescentRate>1</gpxtrkx:AvgDescentRate>
<gpxtrkx:MaxDescentRate>-0</gpxtrkx:MaxDescentRate>
<gpxtrkx:Calories>540</gpxtrkx:Calories>
</gpxtrkx:TrackStatsExtension>
</extensions>
This is what BaseCamp shows, when reading gpx with above statistics:

Ok, it is weird, but one can accept, that BaseCamp doesn't analyze data but shows embedded summary as a true value.
My next step was to remove statistic from gpx and compare recalculated data. To my surprise, BaseCamp showed the same values. I deleted all gpx and imported gpx without statistic again. I got the same statistic again. Finally i have read gpx on other PC, where I got true results:

I think track data can remain somewhere in BaseCamp libraries, I got at least good result when deleting tracks with right click and "delete".