CPU utilization measurment failure

Hello,
I am working on a STR9 scheduler (it is open source - will post a link here when it is ready on the condition you help me to solve this problem :-) )
I don't understand why my code to measure the CPU utilization does not work. It is very simply really: I have an idle task that counts as fast as possible (with the scheduler disabled) upon startup for one second. It saves the accumulated value, then the scheduler is re-enabled and the count begins again, evaluated every second. The ratio between the initial accumulated value and the once per second generated count is the CPU utilization - but the amazing thing is that the one per second count, with the scheduler enabled, tends to the bigger than the initial count (with the scheduler disabled) !
I am quite sure there is no overflow involved - it goes wrong even if I make a measurement every 300 milliseconds (as demonstrated below). Any ideas/recommendations?

// idle task. always executed, and scheduled first
static void idle_task(void)
{
        int32u  l_scheduler_state ;
        int32u  l_counter = 0 ;

        scheduler_disable(&l_scheduler_state) ;

        while (1)
        {
                ++l_counter ; // it is faster to count into a register-allocated variable

                // and later assigned the accumulated value into a static

                if (timer_poll(IDLE_TASK_TIMER, MILLISECONDS_X_10(30), 0) )
                {
                        s_performance_counter_init = l_counter ;
                        l_counter = 0 ;

                        break ;
                }
        }

        scheduler_restore(l_scheduler_state) ;

        while (1)
        {
                ++l_counter ;

                if (timer_poll(IDLE_TASK_TIMER, MILLISECONDS_X_10(30), 0) )
                {
                        s_cpu_utilization = 100 - ( (l_counter * 100) / s_performance_counter_init) ;
                        l_counter = 0 ;

                        debug_message("cpu load %d%\n", s_cpu_utilization) ;
                }
        }
}