unexcpected output about c-thread-pool HOT 1 CLOSED

#include <sys/time.h>

double startTime;
double wallTime(void) {
    struct timeval tv;
    gettimeofday(&tv,NULL);
    return tv.tv_sec + tv.tv_usec/1e6 - startTime;
}

void task(void *arg){
	int sleep_time = rand()%1000000+1000000;
	printf("%5.2f: Thread #%p working on %3ld (%.2f)\n",
	       wallTime(), (void *)pthread_self(), (intptr_t)arg, sleep_time/1e6);
	usleep(sleep_time); // intensive task
	printf("%5.2f: Thread #%p finished on %2ld (%.2f)\n",
	       wallTime(), (void *)pthread_self(), (intptr_t)arg, sleep_time/1e6);
}

 0.00: Thread #0x700001bd6000 working on   0 (1.87)    <-- 1.87 sec task starts on thread 0x700001bd6000
 0.00: Thread #0x7000018c4000 working on   2 (1.59)
 0.00: Thread #0x700001c59000 working on   3 (1.64)
 0.00: Thread #0x700001a4d000 working on   1 (1.26)
 0.00: Thread #0x700002177000 working on   6 (1.53)
 0.00: Thread #0x700001cdc000 working on   4 (1.74)
 0.00: Thread #0x7000019ca000 working on   8 (1.03)
 0.00: Thread #0x700001d5f000 working on  10 (1.63)
 0.00: Thread #0x700001b53000 working on   9 (1.97)
 0.00: Thread #0x700001ad0000 working on   7 (1.32)
                  .
                  .
                  .
 1.64: Thread #0x700001c59000 working on  34 (1.26)
 1.75: Thread #0x700001cdc000 finished on  4 (1.74)
 1.75: Thread #0x700001cdc000 working on  35 (1.83)
 1.78: Thread #0x7000020f4000 finished on 11 (1.78)
 1.78: Thread #0x7000020f4000 working on  36 (1.67)
 1.86: Thread #0x700001f6b000 finished on 17 (1.86)
 1.86: Thread #0x700001f6b000 working on  37 (1.64)
 1.87: Thread #0x700001bd6000 finished on  0 (1.87)    <-- 1.87 sec task finishes on thread 0x700001bd6000
 1.87: Thread #`0x700001bd6000 working on  38 (1.81)   <-- new (1.81 sec) task starts immediately on this thread
                  .
                  .
                  .
 3.58: Thread #0x700001cdc000 working on  58 (1.91)
 3.67: Thread #0x700001947000 finished on 41 (1.41)
 3.67: Thread #0x700001947000 working on  59 (1.70)
 3.68: Thread #0x700001bd6000 finished on 38 (1.81)    <-- 1.81 sec task finishes at 1.87+1.81 = 3.68 elapsed seconds
 3.68: Thread #0x700001bd6000 working on  60 (1.20)
 3.88: Thread #0x700001e65000 finished on 43 (1.20)
 3.88: Thread #0x700001e65000 working on  61 (1.30)