Why do these benchmarks NOT saturate the CPU.

Question

I ran two trivial benchmarks in Powershell and C. I ran this on reasonably well endowed i7-2600 with 16 Gig Win 7 Pro.

Both ran using two of the 8 cores (really 4 cores dual hyper threaded) and ran very peaky ranging over 40% to 70% utilization over its run time (30 to 60 seconds). The rest of the system was quiet but on a domain and both dev environments were open and so was Firefox. The background activity before and after the runs was using 1.5% to 2.5% of the total CPU availability.

What is going on in windows that limits these compute bound processes from pinning the CPU on at least the cores they run on.

This was for a on the fly demo I did and expected the cpu to pin at 100%.

Powershell

$a = 1  
"Starting Loop"
get-date
DO
{
 $a++
} While ($a -le 50000000)
get-date

C

#include <stdio.h>
#include <time.h>

int main (void) {

  long long a;
  time_t mytime;
  mytime = time(NULL);
  printf(ctime(&mytime));
  /* for loop execution */
  for( a = 1; a < 100000000000; a = a + 1 ){

  }
  mytime = time(NULL);
  printf(ctime(&mytime));

 return 0;
}