这个毫秒数是不准确的,是根据操作系统相关的,不是你机器硬件的问题,要更精确的时间就使用其它的函数,QueryPerformanceCounter等
看看GetTickCount函数的说明,
Remarks
The following table describes the resolution of the system timer.
System Resolution
Windows NT 3.5 and later The system timer runs at approximately 10ms.
Windows NT 3.1 The system timer runs at approximately 16ms.
Windows 95 and later The system timer runs at approximately 55ms.