memory issue on ipod 4G

[blackscepter]

i have an iPod 4G 20GB and last night i update the songs in there and the totla of the songs now in my iPod is 2568. these are mp3 and AAC files in 128 kbps.

i understand that the 20GB memory is actually 18.5GB in memory that can used to store file. so after updating song last night i noticed that the memory count is 14.1GB available space when i have 2568 songs in it. i supposed that 2000+ songs should be in 8GB memory consumption already.