Why ga optimization never gives consistent results ?

Hello everyone ! I am looking for some people with experience using ga optimization function.

When I run the optimisation, it never gives me the same results for the variables. And the control variable (to be minimized) never reach the same level.

Do you know what could be the source of error ?

Hints, what I am doing is very simple. I just took my original code where I made my computations, set the variables, set y the final result and in a second script do the ga optimization function as you can see in the screen shot attached.

Thank you very much in advance !

Pierre