But isn’t that why we do TR? Do be in “zones” that target certain energy systems?
Who is this “we”?
PPP: They are called levels and not zones for a reason.
PPP: The training levels are descriptive, not prescriptive.
The misconception that erg mode micromanages your power certainly adds to it.
Conversely, the same misconception leads to inappropriate criticism of erg mode.
Don’t chase average watts. Ride within the watts you’re supposed to aim for, if its 140w, anything between 120 and 160w might be fine 
Most important thing is to keep steady, and not start going to 200+ watts just because the road isn’t dead flat, or there is headwind, or whatever other reason.
Just ride steady, take it easy on the descents.
Also, keeping steady power is not as important as you’d think. It way more important to not ride too hard, than it is to accidentally ride a bit too easy, especially when just doing endurance.
I gave up for endurance, for that I just use heart rate but for longer suprathreshold effort (like vo2max intervals) power is a good thing to have.
Power is also good to track progress as over time you’ll see that for a given heart rate you can generate more power
Thinking about the maths, if raw is REALLY raw, then 3s should have ~20% ripple due to the fact that humans do a lot more pushing than pulling. I hope that the “doubling” of single side is phase shifted to go with missing leg, but it would be easier for garmin/favero to x2, in which case 90 rpm gets you either 4 or 5 pulses in the 3 s rolling window.