If your computer is doing anything else at the same time, it is going to slow down. If you were copying files, surfing the web, or even if you click on the finder and the program was running in the background.

There is a utility that you can use that is in your Utilities folder in your Applications folder called CPU Monitor. There is another utility called Process Viewer that allows you to see all of the programs that your CPU is required to handle at the same time.

Run the benchmark again with the CPU monitor visible. It should max out the bar.

With Process Viewer and the CPU monitor (with the expanded window) applications open, I ran the quickbench.app in 2.333333325 minutes. The Quickbench.app used an average of 92% CPU the entire time (the rest going to other processes) and only dropped as low as 84.8%. It dropped this low because the Window Manager was using about 8.9% and the loginwindow used 4.5% (why I don't know) plus the process viewer took about 2% or so.

Hopefully this helps you out.
I knew it was something strange about my result.

Although I know that surfing the web, mailing etc are stealing CPU power (I´m not dumb! ;) ) I just booted my machine and ran the program. When I then ran the processviewer I saw that the process "LoginWindow" was taking nearly all the CPU power. (Very strange that this process is even running when you´re logged in) So I killed it, and had to log in again. Presto! Now the benchmark runs at 1,8833 minutes.
Do you remember what optimization level the code was compiled at? If you compile from the command line, you use something like 'gcc -O3 quickbench.c' to do level 3 optimization. If you're using Knighthawk's GUI version, you have to set the level in some ProjectBuilder setting somewhere.