Why do AAC files seem to have an inexact bitrate? I'm using ffmpeg to encode audio files as AAC at 256kbps. However, the actual bitrate of the resulting file is rarely 256. It's usually a number close to 256 like 241, 255, 257, 254. In one case it was 125.

What's going on? Is this is a property of AAC or ffmpeg? If I ask ffmpeg for mp3 at 320kbps I always get 320. I thought for a while that the AAC encoder was using 256 as an upper limit and only using the bitrate it needs but then I got a few 257 files so that can't be it.
Both AAC and MP3 are not always encoded with constant bit rate but they support variable bit rate encoding too.