Set statistics time showing different elapsed_time than dm_exec_query_stats view
What version of SQL Server are you using?  This is/was a known issue and was fixed in SQL 2008 SP1 but wasn't ported back to SQL 2005.

http://connect.microsoft.com/SQLServer/feedback/details/487838/cpu-time-sys-dm-exec-query-stats-vs-set-statistics-time-on