In some cases the default settings for the refinement panel are not suitable. This is also true for the accuracy index which basically defines how many results are considered for the filter options in the refinement panel. Find out how the customize the accuracy index here:
http://www.sharepointanalysthq.com/2011/05/search-refinement-accuracy-index-in-sharepoint-2010/
Thomas Maeder | thomasmaeder.com 