This may be the reason for the difference between power shown Garmin and Intervals:
''So this gets a little complicated, but the difference is due to a couple of things going on. One is that there are radio outages that are causing some of the records to be back filled. The second is that the tools you are using to process the data are using the power values assigned to each record and are not using the accumulated power values. Using the accumulated power values is the better approach. Because of this the tools are subject to how the dropped messages are being backed filled.
The accumulated power record at the start of that #6 30 second window is 199200. The end value is 208219. That is a difference of 9019 kj. Average that over 30 seconds gives 300.63 or 301 watts. This is what the Edge reports.
The last 2 seconds on that lap and the 1st second of the next lap have the same accumulated power value because of an outage.
Using the 1 second power values that have been computed using the backfilled data because of the radio drops gives 9330 kj over 30 seconds or 311 as you get.
For the next 15 second lap the accumulated power starts and ends with 208219, 210262 or 2042 kj over 15 seconds 136.2 watts.
The sum of the individual power records gives 1720 over 15 seconds or 115’’