Question: Time[real] keeps on changeing????

I have a problem i am trying to see how long it takes for my program to give a solution but the time keeps on changeing everytime i press enter. I can't seem to understand why?
 

1 )  restart:
       start:=time[real]();
          f:= x-> x^2-5:
         NR:= proc(f,z,M)
              local a,A:
               a := z:
                for A to M do:
                   a := evalf(a) - (f(a)/D(f)(a))
           end do:
      #printf("The answer for the function %a is %f to 6 d.p",f(x),evalf(a)):
  end proc:
 
             NR(f,1,5);
          finish:=time[real]();
          print(`seconds`,finish-start);

                                  4445.118
                                 2.236067977
                                  4445.189
                               seconds, 0.071

 

2)  restart:
start:=time[real]();
f:= x-> x^2-5:
NR:= proc(f,z,M)
   local a,A:
    a := z:
    for A to M do:
      a := evalf(a) - (f(a)/D(f)(a))
       end do:
    #printf("The answer for the function %a is %f to 6 d.p",f(x),evalf(a)):
  end proc:
 
 NR(f,1,5);
finish:=time[real]();
print(`seconds`,finish-start);

                                  4937.274
                                 2.236067977
                                  4937.326
                               seconds, 0.052
 

Its giving the same answer but why is the time not giving a stable value???