MP3 files of same bitrate and same length have a different size

Question

I'm currently working on some multi-channel audio recording software. The basic goal is to be able to record every input device on a computer. I noticed that when I record for a "long" time (more than 1 hour), the different mp3 files (which represent the different input devices) have the same length (and bitrate settings), but the filesize is not the same (although it's not a huge difference, about 10 KB at most). I would expect that the files must have the exact same size, though I'm not 100% sure of it. Are there any audio experts here on Stack Overflow with some advice?

Answer 1

How do you measure the length of files? 10KB of filesize difference at the bitrate of 320kbps is about one quarter of a second, so it is the expected behavior if your length measurements have the precision of 1 second.