As you know, motor rewinding will reduce motor's efficiency from %10 up to %15. Despite of that, efficiency may improve up to 5% reduction after some time working.
Now, my question is that,
1-why this reduction happen?
2- why this improvement happen?

Hello Seyed.
There are many generalisations about the effects on motor efficiencies are after rewinding, and to state that reductions are standard is incorrect.
The best literature to read up this material - in my opinion - is EASA's study on this. A Google search under 'EASA effects on rewinding on motor efficiency' will guide you to a PDF report that is both objective and very interesting.
Take care.
Robert.

I have come across cases where it is reported that the efficiency of rewound motor was found better the original motor of good make.

Agree with Robert, and have found that the upgrades in electrical wiring used for the rewind, or even a reduction in the length of the same size wire will indeed change the resistance and other energy characteristics of the motor. For example, we used to dewind a couple of turns on our HP60 electric slot car motor to lower resistance (and draw more juice), for faster operation around the track.