I set FPS to 120, encode 240 frames, and the video plays back over some 8-10 seconds

I=B4m using the xvid encoder in non-realtime to encode frames, but its like=
 xvid ignores the FPS I set, and records at the incoming speed.

I set the FPS to 120, then encode 240 frames, i.e, 2 seconds of video, put =
the playback file is wrong, and runs at around the same speed I sent the fr=
ames in.

Why would this happen?