Hi David,
Did you already had a closer look?
I noticed that all my ride activities on intervals give somehow lower power values than Garmin does.
For example yesterday activity (lap button pressed once the race started, total lap time: 1h12m): 273w average and 295w normalized. While Garmin gives 288w and 309w. The Garmin values are also the same values that I saw on my bike computer during the race, therefore I tend to think the Garmin values are ok and that somehow Intervals is going funky town on the power averages.