AWK loops test on local vm - bash

Question

I'm trying to understand this difference of 5 seconds between 1000 and 10000 loops extracted from gawk article. Would appreciate your help.
#!/usr/bin/gawk -f
# how long does it take to do a few loops?
BEGIN {
LOOPS=100;
# do the test twice
start=systime();
for (i=0;i<LOOPS;i++) {
}
end = systime();
# calculate how long it takes to do a dummy test
do_nothing = end-start;
# now do the test again with the *IMPORTANT* code inside
start=systime();
for (i=0;i<LOOPS;i++) {
# How long does this take?
while ("date" | getline) {
date = $0;
}
close("date");
}
end = systime();
newtime = (end - start) - do_nothing;
if (newtime <= 0) {
printf("%d loops were not enough to test, increase it\n",
LOOPS);
exit;
} else {
printf("%d loops took %6.4f seconds to execute\n",
LOOPS, newtime);
printf("That's %10.8f seconds per loop\n",
(newtime)/LOOPS);
# since the clock has an accuracy of +/- one second, what is the error
printf("accuracy of this measurement = %6.2f%%\n",
(1/(newtime))*100);
}
exit;
}
[root#krislasnetwork ~]# ./testSpeed.sh
1000 loops took 2.0000 seconds to execute
That's 0.00200000 seconds per loop
accuracy of this measurement = 50.00%
[root#krislasnetwork ~]# vi testSpeed.sh # changing loops from 1000 to 10000
[root#krislasnetwork ~]# ./testSpeed.sh
10000 loops took 15.0000 seconds to execute
That's 0.00150000 seconds per loop
accuracy of this measurement = 6.67%
It appears that each loop during the 10,000 loops is executed faster than each loop during the 1,000. Can you help me understand this behavior?