CI
 

in descent or approach, a CI change does not restart a computation.
i want to know the reason!

What do you mean by computation?
A sentence ending by a 'please' or 'thank you' will generally get your question answered.:E

Engines are generally at idle in decent and once on approach the fuel consumption isn't gonna affect the overall profitability of the flight.

It's more important to conserve fuel while in cruise for hours and hours than in the final minutes of the flight.

That's why CI is irrelevant at that point. Any savings would be too little, too late.