[FFmpeg-user] Problem about duration value of converted mp3 file

[myounggun jang]

When converting a wav file to MP3 using the default option, an error occurs
in the length.
Converted using the following command

ffmpeg.exe -i 1.wav 1.mp3

The duration of the original wav is 1:09:30, but the length of the
converted MP3 is 1:07:16.
The length of the file was checked through Windows Explorer and Windows
Media Player.
However, when checking with ocen audio and other software, it is normally
displayed as 1:09:30.
When I tested using the -b:a option, 64k and 96k are converted to the same
length, but there is a problem with 32k and 48k.

ffmpeg.exe -i sample_2.wav -b:a 96k sample_2_96.mp3

In addition, when converting m4a files to MP3, a problem occurs also in 96k.
Please help me on what to fix or give options.