iPhone audio size vs iPad audio size varies using same Songlist

I have a single playlist in the latest iTunes that is synced to both the iPad 64GH 3G and iPhone 4 32GB (CDMA)
They both contain 2366 Tracks and i have convert to 128Kbps selected on both devices.
The ipad shows 9.2 GB Audio Used
iPhone shows 11.2GB Audio Used
No other tracks or albums, playlists, etc are synced to alter the difference as theyre 100% identical on each device but the size it takes up on each device is off by 3GB.
Any rhyme or reason for this to be occurring?
For what it is worth I had to swap my newly purchased iPhone 4 CDMA for a replacement and the original did the same exact thing too.
Thanks in advanced
MacBook Pro iPad 3G Mac OS X (10.6.6)