Передвижение на среднюю величину (поведение странное ))))

#include <Windows.h>
#include <iostream>
using namespace std;
#include "SFML/Graphics.hpp"
using namespace sf;


// Перемещение на среднюю величину 2ух кадров
//
class ClockTme
{
    LARGE_INTEGER ticksBase, freq;


    public:
        Int64 getTicks()
        {
            QueryPerformanceCounter(&ticksBase);
            return ticksBase.QuadPart;
        }

        Int64 getFrequency()
        {
            return freq.QuadPart;
        }

        ClockTme()
        {
            QueryPerformanceFrequency(&freq);
        }
};


class FrameTme
{
    Int64 ticks, ticksPrev, frequency, elapsedTicks, elTmeMicrosec;
    ClockTme cl;

    void setElapsedTicks()
    {
        if(ticks > ticksPrev)
            elapsedTicks = ticks - ticksPrev;
        // произошло обнуление
        // произойдет примерно через 50 лет
        else if(ticks < ticksPrev)
            elapsedTicks = _I64_MAX - ticksPrev + ticks;
    }

    void setElTmeMicrosec()
    {
        elTmeMicrosec = elapsedTicks * 1000000 / frequency;
    }


    public:
        void updateTme()
        {
            ticksPrev = ticks;
            ticks = cl.getTicks();
            setElapsedTicks();
            setElTmeMicrosec();
        }

        // вызывать после обновление времени updateTme()
        Int64 getElapsedTicks()
        {
            return elapsedTicks;
        }

        // вызывать после обновление времени updateTme()
        Int64 getElTmeMicrosec()
        {
            return elTmeMicrosec;
        }

        void restartTme()
        {
            ticks = cl.getTicks();
        }

        FrameTme()
        {
            frequency = cl.getFrequency();
            ticks = cl.getTicks();
        }
};


struct State
{
    float x, y;
};


class midStateSprite
{

    State prevState;
    State currState;

    float midX, midY, velX, velY;

    public:
        Texture tx;
        Sprite sp;

    midStateSprite(char* txFile, float velocityX, float velocityY)
    {
        tx.loadFromFile(txFile);
        sp.setTexture(tx);
        currState.x = sp.getPosition().x;
        currState.y = sp.getPosition().y;
        prevState = currState;

        velX = velocityX;
        velY = velocityY;
    }

    void setTexture(char* txFile)
    {
        tx.loadFromFile(txFile);
        sp.setTexture(tx);
    }

    void updateStatesAndMove(Int64 &elTmeMicros)
    {
        prevState = currState;
        currState.x += velX * elTmeMicros;
        currState.y += velY * elTmeMicros;
        midX = (prevState.x + currState.x) / 2;
        midY = (prevState.y + currState.y) / 2;
        sp.setPosition(midX, midY);
    }
};


void main()
{
    int wWid = 600, wHei = 500;
    RenderWindow wIn(VideoMode(wWid, wHei), "My12345", Style::Default);

    FrameTme frTm;
    Int64 elTmeMicros;

    midStateSprite mStSp("tree.png", 0.0001f, 0);

    frTm.restartTme();
    Event ev;
    while (wIn.isOpen())
    {
        while (wIn.pollEvent(ev))
        {
            // Close window : exit
            if (ev.type == Event::Closed)
                wIn.close();
            if (ev.type == Event::KeyPressed)
            {
                if (ev.key.code == Keyboard::Escape)
                    wIn.close();
                else if (ev.key.control && ev.key.code == Keyboard::C)
                    wIn.close();
            }
        }

        wIn.clear();

        frTm.updateTme();
        elTmeMicros = frTm.getElTmeMicrosec();
        if(elTmeMicros > 25000)
            elTmeMicros = 25000;

        mStSp.updateStatesAndMove(elTmeMicros);

        wIn.draw(mStSp.sp);
        wIn.display();

        // убираю нагрузку с процессора
        sleep(microseconds(1000));
    }
}