Many thanks for your suggestion and support for TrainingPeaks Virtual.

This difference happens as a result of the accuracy of TrainingPeaks Virtual compared to Strava - specifically, TrainingPeaks Virtual uses the exact height for any elevation gains, whereas Strava uses data rounded to the nearest meter.

This means that, for example, if you were to go up a small 25cm lump in the road four times, TrainingPeaks Virtual would count it as 1m elevation gain (4x0.25 = 1), but Strava would count it as 0 elevation gain (4x0 = 1) purely because of the difference in rounding.

Whilst this might not seem very much, over a long ride those differences gradually add up to give the overall difference in values that you see.