Intervals.icu does activity analysis using a “fixed_watts” stream which is the power stream with short drop outs (8s or less) fixed and power spikes removed.
The average power for the activity reported in the summary section at the top (226w) is Joules / recording time (if available) otherwise elapsed time.
- Joules (3292103 J): The sum of the power stream interpolated to 1s per point
- Recoding time (14593s): The elapsed time (15445s) for the activity with gaps (not recording e.g. paused) of more than 30s removed
So it is your average power when you were actually riding (226w = 3292103 / 14593).
The average power reported by the power curve for the activity is 214w for 4h15m. This is the Joules divided by the elapsed time i.e. it includes all stops. So if you are interested in the “true average power” then this is where to look.
The average power for each interval is the average of all the power points in the interval. So it excludes stops.