Нестабильность значения Hz с вставкой на ассм

function GetCPUSpeed(): string;
const DelayTime = 500;
var TimerHi : DWORD;
     TimerLo : DWORD;
     PriorityClass : Integer;
     Priority : Integer;
begin
  PriorityClass := GetPriorityClass(GetCurrentProcess);
  Priority := GetThreadPriority(GetCurrentThread);
  SetPriorityClass(GetCurrentProcess, REALTIME_PRIORITY_CLASS);
  SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_TIME_CRITICAL);
  Sleep(10);
  asm
    DW 310Fh // rdtsc
    MOV TimerLo, EAX
    MOV TimerHi, EDX
  end;
  Sleep(DelayTime);
  asm
    DW 310Fh // rdtsc
    SUB EAX, TimerLo
    SBB EDX, TimerHi
    MOV TimerLo, EAX
    MOV TimerHi, EDX
  end;
  SetThreadPriority(GetCurrentThread, Priority);
  SetPriorityClass(GetCurrentProcess, PriorityClass);
  Result := 'Частота: '+FloatToStr(TimerLo / (1000.0 * DelayTime))+' Hz';
end;