The methodology used is explained in this pdf
This year, Belgium dropped from the 15th to the 25th position.
The negative result mainly comes from the fact Belgium's policy was judged to be catastrophical (5Oth position on 57 countries)
Germanwatch, as to be expected from a NPO, has a very harsh conclusion :
The Climate Change Performance Index therefore does not have any winners this year. Due to the lack of will to engage themselves more strongly to avoid dangerous climate change, none of the countries achieved positions one to three. Position four to ten of this year are made up by Sweden, Germany, France, India, Brazil, the United Kingdom and Denmark.The last three positions are for Saudi-Arabia, Canada & the USA
The report isn't very hard science, but does give some rough overview of how different countries are acting. It can be found here (pdf)