question about extract frames

[zlxhhh]

when i extract frames video v_Mzojo2EeWu8, segment 3, frame_id 8. I use code from #1 (comment). But i just get 8 pictures(frame_id 0-7).The frame labeled is the ninth picture.
I guess the reason is that the time of segment3 is too short, so how do I get the picture of the ninth frame? thanks a lot