ffmpeg lower the total bitrate after extract

[UnlikeME]

Hello im new in here, i hope somebody has a anwser for me.
i want to check with your guys to see if i do the right way, because the total bitrate getting lower. And after the extract i cant see the video bitrate with mediainfo

Feralserver

ffmpeg version N-89882-g4dbae00bac-static

i have a UHD 4k HDR movie that i have extract, TrueHD - AC3 - Video by this command

ffmpeg -i input.mkv -map 0:0 -map 0:1 -map 0:2 -c copy encode.mkv

Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, bt2020nc/bt2020/smpte2084), 3840x2160 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 23.98 tbc (default)
Stream #0:1(eng): Audio: truehd, 48000 Hz, 7.1, s32 (24 bit) (default)
Stream #0:2(eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 640 kb/s

Then i add the subtitles.srt to the same dir and rund

ffmpeg -i encode.mkv -i eng.srt -map 0:0 -map 0:1 -map 0:2 -map 1:0 -c copy encodefinish.mkv

but what i dont understand, is when i use the first command to extract video and audio, the total bitrate has been a but lower then the originale file?

originale: 56.6
Extract: 56.0

[LigH.de]

The original MKV had a bitrate of 56.6 ... which unit? Mbps, probably? So it lost about 600 kbps. Did it contain any other streams you did not copy? Or did the video possibly contain some stuffing not to get below a minimum bitrate ... depends on how the original MKV was created, which additional data it contains from a really original source.

[UnlikeME]

Yes, 56.6 Mbps. Yes, it contains subtitles .pgs and chapter.

i can see that the bitrate one truehd and ac3 is the same and filesize to. it just wonders me that I cant see the bitrate of the movie not even with FFmpeg -i input.mkv |& grep Stream or mediainfo with rutorrent.

Cant it be the subtitles can make the difference?

[Selur]

Question is does the file size change accordingly, if not then MediaInfos estimation of the bitrate is simply wrong. (not that big of a deal not that uncommon)

[UnlikeME]

Org: 41.2 GB
Rip: 40.8 GB

Only subs extract.

i have just look at one PGS subtitles,

NUMBER_OF_BYTES : 17437857

That is about 16.6 MB

There are 30 subtitles.
16x30 = 1.6GB but some subtitles are less site and maybe som are bigger. idk. But maybe is

[Selur]

30 picture based subtitles + may be some header optimization can account for 400MB
Also 30*16.6 = 498 not 1600.