Refresh Rate Change

Hi

Does anyone know why XBMC changes the Refresh Rate on a 480p MKV first from 1080p 50Hz to 1080p 60Hz and then back to 1080p 50Hz XBMC is set to display the UI in 50Hz... on HD MKV Material it changes direct from 50Hz to 24Hz... but on a lot of dvd rips in SD it changes first to 60 and then back to 50 Hz??

My Xorg.conf

http://pastebin.com/t895u6Jy

and a Media Info of one of the files...

http://pastebin.com/FMASH24S

System is a ION2 Nettop. GT218, Atom D510