（非vbr）mp3文件的长度（时间）

Im currently researching the mp3 format in order to build an mp3 decoder. After some thinking I figured out that the simplest way to calculate the length of the song would be to divide the size by the bitrate (taking in account the size of the ID3 tag etc.), and transform the result to minutes. Using this method on a few songs I got accurate times. I always assumed the time of the song is the length of the pure audio data, but in this method, frames are also "considered" part of the song (when calculating the time). Also, the I understood that the audio data in the mp3 file is compressed, so when its decompressed the size of it will be larger of course, and then the time calculation seems un accurate. Am I missing something here? because it just doesnt make any sense to me that the songs length is calculated with the compressed data and not the uncompressed ones, and frames which are a DWORD each are not ignored.Im currently researching the mp3 format in orde