If that is your post-workout chart it looks like you have a trainer with power smoothing enabled. You may want to disable that to see actual fluctuations generated during workouts.
May not address your question but you’ll get a more accurate representation of what occurs.
Related –
Focus duration seeming off - Support - Xert Community Forum (xertonline.com)
I think the other reason may be the difference in type and duration –
4x30 sec @ 4 min MMP – flat (why does your chart show 3x in second set?)
1x1 min @ 75% PP MPA – flat
7x20 sec @ 75% PP MPA – not so much 
If you turn off ERG smoothing those 20 sec intervals will likely show if trainer response time is a factor. Since SMART intervals rely on actual power generated, I would think ERG smoothing would skew the in-ride calculation/adjustment. But does that mean better or worse?
In either case the difference appears minor, so you got the intended strain ratio work done. 