I know its another year later but I had to say that Shailesh's post is incorrect in MDM 7.1.
A field that is set to type Auto ID always increments by +1 and never revisits an old Auto ID even if you deleted all the records.
Internal ID on the other hand takes the first available internal ID in that table, so if you delete a record, its internal ID becomes available for use again.
If you create records which have an Auto ID field from scratch and never delete any records, then your Auto ID and internal ID will be in sync but otherwise not.
Yassir