Amazing but unexpected behavior of a simple C test

[Cef]

Hello

Some time ago I made this simple C speed test:

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

extern double sqrt(double);

void compute( void ){
long n;
double k;

for(n=1;n<750000000;++n){
k = n * 3;
k = sqrt(k) / 2;
}
printf("Test ended\n");
return;
}

void main()
{
clock_t start_time, end_time;
double Time, S,E,Tics;

start_time = clock();

compute();
end_time = clock();
S = start_time;
E = end_time;
Tics = CLOCKS_PER_SEC;
Time = (E - S) / Tics;
printf( "Execution time was %f seconds\n", Tiempo) ;
}

At that time I had the Pentium Dual-Core E5400 2.70GHz. The
time execution was 21.20 seconds. What amazed me was that
I ran it in four Linux (Ubuntu) terminals simultaneously
and the times where: 21.20, 21.17, 21.28 and 21.22 seconds.
Question ONE: Why are those times equals or almost equal to
the test I ran in just one terminal alone?

Recently I upgrade to an Intel Core i3-2120 3.30GHz and the
time in one terminal was sightly more: 23.39 seconds.
Question TWO: Why, if this microprocessor has a faster clock
and better technology.

Running this test in four terminals simultaneously give the
expected result to me, longer times because the OS has to
give slides of power to each terminal, about 34 seconds each one.
However, when I disabled the Hyper Threading feature from
the BIOS (the MB is an Asrock H67M-GE motherboard), the four
terminal test behave like the E5400, that is, all the times
around 23.39 seconds.
Question THREE: Did you expect this? Why?

Thank you very much in advance.

Regards,

Carlos