Using mongodb 2.4.9 ( database created since 1.8.3 ).

I read an anomaly in the object count between a replicaset's primary and secondary collection; even though the secondary is not behind the Primary.

Comparing with
$ db.master.chunks.stats()

Secondary 1 has 7 document more than the Primary and Secondary 2.
A reindex operation does not change that difference.

An export of all _id fields from secondary 1 produces a list that matches the number of expected objects: 6087983

What are these 7 extra objects about ? System related documents maybe ? Damaged documuments ? How do I find out ?