pgbench results arent accurate

Hey,
I installed a new postgres 9.6 on both of my machines. I'm trying to
measure the differences between the performances in each machine but it
seems that the results arent accurate.
I did 2 tests :

1)In the first test the scale was set to 100 :
pgbench -i -s 100 -U postgres -d bench -h machine_name
pgbench -U postgres -d bench -h machine_name -j 2 -c 16 -T 300
RUN TPS - machine1 TPS-machine2
1 697 555
2 732 861
3 784 842

2)In this test the scale was set to 10000 :
pgbench -i -s 10000 -U postgres -d bench -h machine_name
pgbench -U postgres -d bench --progress=30 -h machine_name -j 2 -c 16 -T
300
RUN TPS-MACHINE1 TPS-MACHINE2
1 103 60
2 63 66
3 74 83
4 56 61
5 75 53
6 73 60
7 62 53

In both cases after the initalization I restarted the database and cleared
the cashe(echo 1 > /proc/sys/vm/drop_caches) one time. During all the runs
I didnt shutdown the machine.

Now, I was hopping the the tps will be almost the same in each machine for
all the runs. In other words, I wanted to see that the tps in machine1
during all the tps are almost the same but I see that the values arent
accurate.

Any idea what might cause the differences in every run ?