Its unfortunate, but clicking on the "Custom" column will always sort the listed values alphabetically! This is also true when you format the values as a number like... <(#System.Video.EncodingBitrate)> Im not know if the programmers could change this when "Custom" only uses... <(#SomeProperty)>
There is a setting for "Display Settings, Sorting, Logical Sorting" to sort any of the numbers numerically, instead of alphabetically. So maybe the programmers could change this setting, so that it also conducts sorting for the "Custom" column ?