A confusion about a mkv video which missing bit_rate information

[pixeldin]

Hello community, I try to cut a certain segment of an input.mkv video and this software works perfectly.
And now I had some extra cut needed so I check some command from developer mode for reference. In a substep of cutting like this:

ffmpeg -hide_banner -ss '1800.00000' -i 'input.mkv' -ss 0 -t '0.79863' -map '0:0' '-c:0' h264 '-b:0' 1503233 -map '0:1' '-c:1' copy -ignore_unknown -video_track_timescale 1000 -f matroska -y 'input-smartcut-segment-encode-0.mkv' 

In this step, the parameter '-b:0' used as 1503233, can someone please tell me how this value is obtained? I know that -b:0 is an option in the FFmpeg command line parameters to set the bitrate of the video.
What I try to do is use ffprobe obtain the bitrate of the original video by

ffprobe -v error -select_streams v:0 -show_entries stream=bit_rate 
-of default=noprint_wrappers=1:nokey=1 input.mkv

and the output info is:
N/A

It seems that the bit_rate is not stored in the original mkv file information. If the bit_rate value can be calculated by other metric shown in ffprobe, please let me know :)

[pixeldin]

It looks fine so far, I just found the answer in the source code.

lossless-cut/src/smartcut.js

Line 46 in 9130924

videoBitrate = stats.size / videoDuration;

Thank you community, leave it here to those in need :)

[mifi]

Hi! this is because you're using "smart cut" so it will try to match the bitrate. if you disable smart cut there will be no encoding/bitrate calculation