A cut you did not re-encode landed somewhere else.

A stream is not a pile of frames. It is a sequence of groups, and only the I-frame at the head of each one can be decoded on its own — every other frame is a description of how the picture differs from one or two others. So a trim that copies rather than re-encodes cannot start where you asked. It starts at the nearest preceding I-frame, because that is the only place a decoder can begin.

The engine is free and needs no account. At a 250-frame keyframe interval and 25 fps that is one every ten seconds, so a cut asked for at 3.2 s lands at 0.0 s — and the worst it could be is (250 − 1) / 25 = 9.96 seconds. Nothing warns you: the file is valid and the cut is real. And only one end moves — an out-point needs no keyframe at all, because stopping is free — so the head is quantised, the tail is exact, and the clip comes out longer than it was asked to be. Underneath that, the frames are not even stored in the order you see them: a B-frame needs a picture from later, so that picture is stored earlier, and the decoder holds it before it can show anything. Every lane ships a worked example that costs nothing. Writing the report needs credits.