WH_MOUSE_LL Hook event timing mystery

I have set a WH_MOUSE_LL Low Level Mouse Hook to intercept wheel movements. It's generally working fine, but I am getting odd results when timing the ticks or ms between each HookCallback. This is what I get when moving the wheel progressively from slow to very fast:

// # of ticks elapsed between mouse wheel events:
112180
37428
56167
37436
37454
37428
37501
18550
37547
18722
37432
18720
18861
39636
18755
18324
18753
18707
18863
18582
18695
18713
// etc..

So basically what I am getting here are multiples of ~18700, not an expected linear progression. Translating these values in ms, I get 40, 24, 16 and 8 ms (8 is the shortest time possible as mouse frequency is by default 125 hz). I need very precise timings for my application.

Here's the code in C# (minus UI):

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr SetWindowsHookEx(int idHook,
        LowLevelMouseProc lpfn, IntPtr hMod, uint dwThreadId);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool UnhookWindowsHookEx(IntPtr hhk);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode,
        IntPtr wParam, IntPtr lParam);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr GetModuleHandle(string lpModuleName);

    [DllImport("kernel32.dll")]
    private static extern bool QueryPerformanceCounter(out long lpPerformanceCount);

    [DllImport("kernel32.dll")]
    private static extern bool QueryPerformanceFrequency(out long lpFrequency);

    private long timerFrequency = 0;
    private long lastWheelTick = 0;
    private LowLevelMouseProc _proc;
    private static IntPtr _hookID = IntPtr.Zero;
    private List<long> log;

    public run()
    {
        log = new List<long>();
        _proc = HookCallback;
        QueryPerformanceFrequency(out timerFrequency);
        _hookID = SetHook(_proc);
    }

    private static IntPtr SetHook(LowLevelMouseProc proc)
    {
        using (Process curProcess = Process.GetCurrentProcess())
        using (ProcessModule curModule = curProcess.MainModule)
        {
            return SetWindowsHookEx(WH_MOUSE_LL, proc,
                GetModuleHandle(curModule.ModuleName), 0);
        }
    }

    private delegate IntPtr LowLevelMouseProc(int nCode, IntPtr wParam, IntPtr lParam);

    private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0 && MouseMessages.WM_MOUSEWHEEL == (MouseMessages)wParam)
        {
            long wheelTick = 0;
            QueryPerformanceCounter(out wheelTick);
            long diff = wheelTick - lastWheelTick;
            long RPM = (timerFrequency / diff) / 60;
            lastWheelTick = wheelTick;
            log.Add(diff);
        }
        return CallNextHookEx(_hookID, nCode, wParam, lParam);
    }

    private const int WH_MOUSE_LL = 14;

    private enum MouseMessages
    {
        WM_LBUTTONDOWN = 0x0201,
        WM_LBUTTONUP = 0x0202,
        WM_MOUSEMOVE = 0x0200,
        WM_MOUSEWHEEL = 0x020A,
        WM_RBUTTONDOWN = 0x0204,
        WM_RBUTTONUP = 0x0205
    }

There's no other processing between mouse events. Also tried using Stopwatch (ticks and ms) with same results.

Any thoughts as why this may happen? Thanks!