different getduration values for same length movies h264 and prores422

hi maxers, my first post, although I've been up and down the forum for a few months now...

I have been making a few players which push out pro.res.422 video and 8 channels of audio, direct to window at 2560px with good results.

I use a max interface with a midi synth to start / stop / go to next clip, in conjunction with showing h264 preview versions of the same clips as the main output.

An editor renders out the big 422 files out of FCP and I use quicktime to make smaller h264 preview clips (at "current" frame rate and key frames set to automatic, same duration)

now, when I do a getduration on both clips (both the 422 and the h264 preview) I sometimes get back different frame counts, whilst quicktime tells me the clips are the same length. the occurring problem does not allow me to scrub through the both the clips using a single slider.

So what could cause discrepancies in frame count between the two video formats whilst quicktime says they are exactly the same length? Anyone have any ideas....???

here is a patch (basically) showing the mechanism I am using:

thanx for any help,
carlitos