Incorrect result when using regexp in $in

Hi,

I am operating a 6 shard MongoDB Cluster.
The following two patterns of queries should have the same results, but sometimes they are incorrect.

mongos> db.programs.find({ _id: /hoge/ }).count()

0

mongos> db.programs.find({_id: {$in: [/hoge/] }}).count()

51

However in explain("executionStats") results both nReturned is 51.
Also, even I execute the same query on each shard node, each result is different.

What kind of cause can be considered?