Converted MKV to MP4 container - need help understanding changes

[kendra69]

So I had an mkv file, which I converted to mp4 using the command
>ffmpeg -i baby_cakes.mkv -codec copy baby_cakes.mp4

Info obtained from ffprobe for mkv file:
encoder : libebml v1.3.4 + libmatroska v1.4.5
Duration: 01:19:52.54, start: 0.000000, bitrate: 1747 kb/s
Stream #0:0: Video: h264 (High), yuv420p(progressive), 640x480 [SAR 1:1 DAR 4:3], 24 fps, 24 tbr, 1k tbn, 48 tbc (default)
Stream #0:1(eng): Audio: mp3, 24000 Hz, stereo, fltp, 64 kb/s (default)

Info obtained from ffprobe for mp4 file is:
encoder : Lavf58.29.100
Duration: 01:19:52.54, start: 0.000000, bitrate: 1752 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x480 [SAR 1:1 DAR 4:3], 1682 kb/s, 24 fps, 24 tbr, 16k tbn, 48 tbc (default)
Stream #0:1(eng): Audio: mp3 (mp4a / 0x6134706D), 24000 Hz, stereo, fltp, 64 kb/s (default)

So I noticed there is a slight increase, (mkv) 1,022,545 KB vs (mp4) 1,025,022 KB

Q1. Why has the video stream increased in bitrate, from 1747 kb/s vs 1752 kb/s ?

Q2. What does the numbers for `tbr` mean ? The change is 1k tbn (mkv) vs 16k tbn (mp4)

[ProWo]

The single streams are exactly the same.
The differences are due to different headers and metadata. You can test this by muxing the mp4 back into an mkv.

[Selur]

tbn = the time base in AVStream that has come from the container
tbc = the time base in AVCodecContext for the codec used for a particular stream
tbr = tbr is guessed from the video stream and is the value users want to see when they look for the video frame rate
see: http://ffmpeg-users.933282.n4.nabble.com/What-does-the-output-of-ffmpeg-mean-tbr-tbn-t...-td941538.html