ffmpeg timestamps and mpv timestamps do not match

[pikim]

I would like to cut a h264 video using SmartCut. The first frame the output should contain has the timestamp 327.76 in Vidcutters preview. When I try to export this frame using ffmpeg -ss 327.76 -i movie.avi -frames:v 1 out0.jpg I get an image of the frame that comes 2 frames before the desired one.

So there's an offset of two frames after about 3 minutes. The bigger the timestamps become the bigger the offset becomes. It doesn't matter whether ffmpeg uses input or output seeking. After about 30 minutes there are 7 frames offset. The ffprobed timestamps match the ffmpeg output.