what i think here , external fragmentation is basically handled by sql server internally and data get inserted in mixed extent initially (i think we can keep "table having identity col's PK" out here ) later on data moves into contigous extent. this could be reason why external fragmentation get avoided.
I welcome comments here 🙂
-------Bhuvnesh----------
I work only to learn Sql Server...though my company pays me for getting their stuff done;-)