gettimeofday函数获得的微秒数是从什么时候开始算的，另为什么usleep(20)前后获得的微秒数之差会大于20，而且不是一个固定值

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

int main()
{

struct timeval before_usleep;
struct timeval after_usleep;


gettimeofday(&before_usleep,NULL);
usleep(20);

gettimeofday(&after_usleep,NULL);
printf("%d,\n%d\n",before_usleep.tv_usec,after_usleep.tv_usec);
printf("sleep time =%dms\n",after_usleep.tv_usec-before_usleep.tv_usec);
exit(0);
}


[kiongf@localhost ~]$ ./a.out
667131,
668155
sleep time =1024ms
[kiongf@localhost ~]$ ./a.out
367426,
368407
sleep time =981ms
[kiongf@localhost ~]$ ./a.out
48116,
49014
sleep time =898ms
[kiongf@localhost ~]$ ./a.out
553446,
553849
sleep time =403ms
[kiongf@localhost ~]$ ./a.out
475882,
476690
sleep time =808ms

这是我多次运行的结果。usleep的差值越来越大从70ms-981ms
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