ffmpeg combine audio/ video output bitrate ratio

I attempted to combine the audio and video using ffmpeg, and I set the output bit rate, for example, ffmpeg -i Testing_6_11_2024_12_13_08_AM.mp4 -b:a some output_video.mp4 -b:v some bufsize some maxrate some
The output file bitrate that I obtained was close to or less than the bitrate that was specified in the command. I’m not able to attain the precise bit rate that I specified in the command.
Is this the expected behavior
e.g.: input video bitrate:50000k
output bitrate mentioned in the command is:20000k
Observed Result : 15000k-17000k