MKVToolnix reduces overall bitrate of videos

[temp22]

I have a bunch of old avis and separate subtitles I wanted to join together with MKVToolnix, which as I understand shouldn't do anything to the input files since it only muxes them. However each file I tried combining with the sub file resulted in a smaller mkv. Mediainfo (and Windows) reports a reduced overall bitrate. How do I prevent that, or is there another alternative program that doesn't muck up my files?

Here's an example, the same thing happens with every file:

[hardy]

Compression is set to "determine automatically"
Select NO and it will not be touched
Adding subtitles will also change the overall bit rate

[jagabo]

Different containers, different overhead. Also, MediaInfo only estimates bitrates.

[temp22]

I set "no extra compression" on all three input files (video, audio, subs) and the mkv file still ends up smaller. I can't spot any distinctive visual degradation (video quality sucks in the first place) and the length is the same as well, so I guess it's the avi overhead?

[Bernix]

In avi it usually take 5MB per hour (overhead) depends on program. From wikipedia

[El Heggunte]

More info /details:

http://www.alexander-noe.com/video/amg/en_estimate_overhead.html

&&

http://www.alexander-noe.com/video/amg/en_overhead_comparison.html