Oracle ASM: Freeing up space

Oracle RAC 11.2 with asm storage, all type = EXTERN redundancy. I've been cleaning up some tablespaces and shrinking datafiles back down to sizes that they're not likely to grow much larger than (processes no longer used had blown them up). The sum of the bytes of datafiles from dba_data_files reflects this shrink, however, USED_MB in v$asm_diskgroup has not fallen and it doesn't match to what adding up the datafiles bytes is? What would account for this difference? We typically use v$asm_diskgroup to monitor if we're running low on space but this doesn't *appear* to be correct. SELECT name group_name , type type , total_mb total_mb , (total_mb - free_mb) used_mb , ROUND( (SELECT SUM(bytes)/1024/1024 FROM dba_data_files WHERE file_name LIKE '+DATA02%' ),0) AS datafile_used FROM v$asm_diskgroup w WHERE name = 'DATA02' GROUP_NAME TYPE TOTAL_MB USED_MB DATAFILE_USED ______________________________________ DATA02 EXTERN 542712 516902 506447