Many indexed queries are bracketed by data type. For example the index bounds for { a:

} are

	"indexBounds" : {
		"a" : [
			[
				0,
				1.7976931348623157e+308
			]
		]
	},

The max value of a we will scan is 1.7976931348623157e+308, not maxKey. When determining which shards a query should be sent to, though, we do not perform type bracketing. The queryutil code implements a special mode that disables bracketing just for this case. It would be helpful to understand why type bracketing is disabled.

    void ChunkManager::getShardsForQuery( set<Shard>& shards , const BSONObj& query ) const {
        OrRangeGenerator org(_ns.c_str(), query, false);

(The 'false' argument disables type bracketing.)