In most cases, ID is the value of the item's Index property when it was first added to the collection. But, unlike Index values, ID values are never changed or reassigned. If an item is removed from a collection, its ID not used again. 

An item's Index may change when other items are deleted from the collection or when items are rearranged. ID provides a unique identifier for each item that is unchanged for the life of the collection. 

Index