请教关于计算CPU占用率的算法的一个疑问，谢谢

    // CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) / NumberOfProcessors
              dbIdleTime = 100.0 - dbIdleTime * 100.0/(double)SysBaseInfo.bKeNumberProcessors + 0.5;看到计算CPU占用率的算法，为何要加0.5呢？是什么意思？麻烦各位指导一下，谢谢附：
"java"
inline int CCpuUsage::OnTimer()
      {
          status = NtQuerySystemInformation(SystemTimeInformation,&SysTimeInfo,sizeof(SysTimeInfo),0);
          if (status!=NO_ERROR)
              return 0;          // get new CPU's idle time
          status = NtQuerySystemInformation(SystemPerformanceInformation,&SysPerfInfo,sizeof(SysPerfInfo),NULL);
          if (status != NO_ERROR)
              return 0;          // if it's a first call - skip it
          if (liOldIdleTime.QuadPart != 0)
          {
                  // CurrentValue = NewValue - OldValue
              dbIdleTime = Li2Double(SysPerfInfo.liIdleTime) - Li2Double(liOldIdleTime);
              dbSystemTime = Li2Double(SysTimeInfo.liKeSystemTime) - Li2Double(liOldSystemTime);                  // CurrentCpuIdle = IdleTime / SystemTime
              dbIdleTime = dbIdleTime / dbSystemTime;                  // CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) / NumberOfProcessors
              dbIdleTime = 100.0 - dbIdleTime * 100.0/(double)SysBaseInfo.bKeNumberProcessors + 0.5;              m_nCpuUsage = (UINT)dbIdleTime;
  }          // store new CPU's idle and system time
          liOldIdleTime = SysPerfInfo.liIdleTime;
          liOldSystemTime = SysTimeInfo.liKeSystemTime;          // wait one second
          return 0;
      }