硬件定时器定时不准确？

static rt_err_t timeout_cb(rt_device_t dev, rt_size_t size)
{
    rt_kprintf("this is hwtimer timeout callback fucntion!\n");
    rt_kprintf("tick is :%d !\n", rt_tick_get());
    return 0;
}
static int hwtimer_sample(int argc, char *argv[])
{
    rt_err_t ret = RT_EOK;
    rt_hwtimerval_t timeout_s;      /* 定时器超时值 */
    rt_device_t hw_dev = RT_NULL;   /* 定时器设备句柄 */
    rt_hwtimer_mode_t mode;         /* 定时器模式 */
    /* 查找定时器设备 */
    hw_dev = rt_device_find(HWTIMER_DEV_NAME);
    if (hw_dev == RT_NULL)
    {
        rt_kprintf("hwtimer sample run failed! can't find %s device!\n", HWTIMER_DEV_NAME);
        return RT_ERROR;
    }
    /* 以读写方式打开设备 */
    ret = rt_device_open(hw_dev, RT_DEVICE_OFLAG_RDWR);
    if (ret != RT_EOK)
    {
        rt_kprintf("open %s device failed!\n", HWTIMER_DEV_NAME);
        return ret;
    }
    /* 设置超时回调函数 */
    rt_device_set_rx_indicate(hw_dev, timeout_cb);
    /* 设置模式为周期性定时器 */
    mode = HWTIMER_MODE_PERIOD;
    ret = rt_device_control(hw_dev, HWTIMER_CTRL_MODE_SET, &mode);
    if (ret != RT_EOK)
    {
        rt_kprintf("set mode failed! ret is :%d\n", ret);
        return ret;
    }
    /* 设置定时器超时值为5s并启动定时器 */
    timeout_s.sec = 5;      /* 秒 */
    timeout_s.usec = 0;     /* 微秒 */
    if (rt_device_write(hw_dev, 0, &timeout_s, sizeof(timeout_s)) != sizeof(timeout_s))
    {
        rt_kprintf("set timeout value failed\n");
        return RT_ERROR;
    }
    /* 延时3500ms */
    rt_thread_mdelay(3500);
    /* 读取定时器当前值 */
    rt_device_read(hw_dev, 0, &timeout_s, sizeof(timeout_s));
    rt_kprintf("Read: Sec = %d, Usec = %d\n", timeout_s.sec, timeout_s.usec);
    return ret;

Read: Sec = 3, Usec = 499387
msh />this is hwtimer timeout callback fucntion!
tick is :38289 !
this is hwtimer timeout callback fucntion!
tick is :43284 !
this is hwtimer timeout callback fucntion!
tick is :48279 !
this is hwtimer timeout callback fucntion!
tick is :53274 !
this is hwtimer timeout callback fucntion!
tick is :58269 !