-ss and -endpos not exact

Dear List,

I'd like to create a film cutting parts from the grabbed file. I grabbed the film
to an avi file (video: ffmjpeg, audio: PCM) with VirtualDub and I wanted to
eliminate the ads.

As a first try I cut the parts with VirtualDub and after that I mencodered them
to one file using 'mencoder -oac copy -ovc copy -forceidx -o result.avi
parts*.avi'. It worked fine.

But after that I grabbed a huge theatre performance in 3 acts and there were no
enough space on my HDD to (almost) duplicate the original ffmjpeg file, so I
decided to just read the necessary parts by mencoder's "-ss HH:MM:SS.mmm" and
"-endpos HH:MM:SS.mmm" arguments. At the end I just concatenated the parts to one
by mencoder.

The start and end positions of the parts were detected by VirtualDub but I
checked them by mplayer (unfortunatelly mplayer does not shows millisencond, so I
just pressed '.' many times to count by how many frames it left the whole
second). The frame times matched exactly.

After I concatenated the parts to one I discovered that at the beginning of all
parts (except the 1st one) there remained 3 frames from the ad.

Could anybody explain why? As I know all frames in an ffmjpeg'd encoded video are
keyframes, so they should by cut exactly. What did I wrong?

Thanx for Your help (and marry Xmas... :-) )!
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