Different file size and bitrate after muxing in MKVmerge

[gtx780]

Hello , i use MKVmerge to mux an audiotrack with a video file, after finishing i noticed that the file size and bitrate of the muxed video is different than the original video.

Original video: 1.41GB
Muxed video: 1.43GB

Original video Bitrate: 1500 Kbps
Muxed video Bitrate: 1440 Kbps

I thought the size and bitrate would be the same after muxing but its not and i dont know why? Can someone explain? Thanks in advance

[jagabo]

Different containers, even different muxers producing the same container, have different overhead. Bitrate is often an estimation by the program reporting it. Programs like MediaInfo don't parse the entire video. They just report what the header says or estimate bitrate based on file size minus audio (an maybe other streams) size.