Regular Data Space Specification vs Text/Image Filegroup
Regular data is in-row data and it is located on the filegroup listed in Regular Data Space Specification, LOB data is located on a Text/Image Filegroup that can be different from a filegroup where in-row data is located.

You specify these filegroups when create a table that contains LOB data:

CREATE TABLE ...
[ ON { partition_scheme_name ( partition_column_name )   
           | filegroup1   
           | "default" } ]   
    [ TEXTIMAGE_ON { filegroup2 | "default" } ] 


Here filegroup1 is for regular data and filegroup2 is for LOB data.


  TEXTIMAGE_ON { filegroup| "default" }
  
  Indicates that the text, ntext, image, xml, varchar(max),
  nvarchar(max), varbinary(max), and CLR user-defined type columns
  (including geometry and geography) are stored on the specified
  filegroup.
  
  TEXTIMAGE_ON is not allowed if there are no large value columns in the
  table. TEXTIMAGE_ON cannot be specified if partition_scheme is
  specified. If "default" is specified, or if TEXTIMAGE_ON is not
  specified at all, the large value columns are stored in the default
  filegroup. 
  The storage of any large value column data specified in
  CREATE TABLE cannot be subsequently altered.


CREATE TABLE (Transact-SQL)

Note the last cited phrase in bold.
If you have LOB data and think that you can just rebuild your table on another filegroup, your LOB data will not be moved at all.