I know VNAV has it's quirks but FWIW I had few thousand hours on the T7 and can't say I ever saw what you describe happen as a matter of routine.

Given old age I can only guess I'd be suspecting maybe winds (the point Uplinker made)/ or, and I'm clutching at straws here - programmed descent speed vs. programmed cruise speed - does it routinely happen if you transition from Econ cruise to Econ descent?

All FADEC engines (in "Normal" mode) adjust the fuel flow to keep the target N1 or EPR - Rolls is no way unique in the that regard. The EAI debit is to protect EGT margins.
However, since anti-ice is normally only used when it's cold - well below the rating corner point for the Takeoff rating range - not everyone bothers to take a performance debit for anti-ice for Takeoff. Max Climb/Max Con are a different issue.
Some models permit dispatch with an EAI valve locked open (which can happen when it's pretty warm), but that's typically handled via the MEL.

ok, thanks.

That's usually when I see it. Winds were my first thought as well, but I'm not sure that's it. "Routine" might be an exaggeration but it certainly hasn't been a one-off. I'll see it a couple of times on a given trip and then not see it for a couple of trips.

B777 stab trim levers
 

What/when are manual stab trim levers on center pedestal for ? They are not mentioned in any non normal checklist. When to use ?

If the dual switches on the yoke failed you would still be able to trim the aircraft.
I imagine not everything is in a checklist and some aspects of the operating the jet require the pilot to, well, be a pilot.

Don't know if this applies to the 777, but on the 787:

"If there is a complete loss of flight control signalling, direct wiring from the flight deck to the stabilizer and a spoiler pair allow pilot control of pitch using the alternate trim switches and roll using the control wheel."

Weird question, but still…
What is exact size of a small pitch rectangle on PFD (in degrees)?
In other words: What will be the pitch angle, if I just touch the 5 degree line with rectangle, then, if I keep inline the upper edge of the rectangle and the 5 degree pitch line?

In other words, you can use them for normal ops(but not allowed when the autopilot is engaged). Try it in the sim sometime.

Vnav influenced by wind and temperature variation?