What factors cause variations in user space instruction counts for the same executable run multiple times on the same machine?

Issue

• What factors cause variations in user space instruction counts perf stat instructions:u for the same executable run multiple times on the same machine?

# cat testcase.c 

#include <stdio.h>
int main(int argc, char* argv[])
{
    int i;
    for (i = 0; i < 100000000; ++i);
    return 0;
}

# for i in {1..10}; do     setarch $(uname -m) perf stat -e instructions:u ./testcase;   done |& grep instructions
       300,075,779 instructions:u            #    0.00  insns per cycle        
       300,075,781 instructions:u            #    0.00  insns per cycle        
       300,075,780 instructions:u            #    0.00  insns per cycle        
       300,075,775 instructions:u            #    0.00  insns per cycle        
       300,075,778 instructions:u            #    0.00  insns per cycle        
       300,075,779 instructions:u            #    0.00  insns per cycle        
       300,075,779 instructions:u            #    0.00  insns per cycle        
       300,075,778 instructions:u            #    0.00  insns per cycle        
       300,075,780 instructions:u            #    0.00  insns per cycle        
       300,075,781 instructions:u            #    0.00  insns per cycle