Re-reading a specific frame using VideoReader returns inconsistent frames

I'm using VideoReader on R2011a to read in a file that has a variable frame rate. My code looks something like this:

f = figure;
frameNum = 3600; %arbitrary frame number
for i = 1:100
   mov(frameNum).cdata = read(vidObj, frameNum);
   imagesc(mov(frameNum).cdata); 
drawnow;
end

This code should read and display the same video frame over the 100 reads. However, when I run it, the picture moves - it does not grab the same frame every time. The read function appears to iterate on every call.

Is this expected behaviour? What can I do to counteract this and ensure that I am grabbing the same frame every time?

Thank you for your help.