Why is my code not returning the memory to computer?

I am trying to use the ipcam package to capture and process some images and record them on disk continuously, but after each loop the program is just consuming more and more memory and I can't make the matlab return it to the system. I already tried to stop it and use the command pack and all the possible variants of clear, nothing returns the memory to the system. The problem can be replicate with this code:

url = 'http://172.18.131.248/axis-cgi/mjpg/video.cgi';
cam = ipcam(url);
img = zeros(1280, 720, 3);
while(true)

    file_name = strcat(datestr(now,30), '.avi');
    outputVideo = VideoWriter(fullfile('./', file_name));
    outputVideo.FrameRate = 15;
    open(outputVideo);
    img = rgb2gray(uint8(snapshot(cam)));

    for offset = 0:450  
        writeVideo(outputVideo, img);
    end

    close(outputVideo);
    clear outputVideo;
end

[Geoff Hayes]

Bruno - please explain why you have a loop that writes the same image 451 times

    for offset = 0:450  
        writeVideo(outputVideo, img);
    end

What is the purpose of this code especially since the offset variable is not being used?