MongoDB $in + sort() query

gistfile1.sh

> db.mydocs.find({_id: {$in: [ObjectId("51be3ca24cc1682a13000003"), ObjectId("51be3c3b2799560d13000003")]}}).sort({'published': 1}).explain();
{
    "cursor" : "BtreeCursor _id_ multi",
	"nscanned" : 2,
	"nscannedObjects" : 2,
	"n" : 2,
	"scanAndOrder" : true,
	"millis" : 0,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		"_id" : [
			[
				ObjectId("51be3c3b2799560d13000003"),
				ObjectId("51be3c3b2799560d13000003")
			],
			[
				ObjectId("51be3ca24cc1682a13000003"),
				ObjectId("51be3ca24cc1682a13000003")
			]
		]
	}
}