Question

Which is the reason for which each export(or bounce) of the same instrument is different from previous?

Example: When i bounce a instrument and immediately afterwards I make another bounce of him (without touch nothing on channel or master or send or insert), the bounces insn't idetical. Indeed If I change the phase of one of this, the sum isn't 0. Why?

Answer 1

Without any further information, this behavior is more than likely due to an FX plug-in such as chorus, flanger, or reverb.  Another possible reason could be the audio interface that you are working with.  Try doing a realtime bounce from Song>Export Mixdown.  Just choose the option for realtime processing and import the bounce back into a track.

Answer 2

Even without any inserts the bounce command of  instruments (for example NI Massive) isn't never extactly the same.