mongodb Compound Index query wierdness.

gistfile1.txt

mydb:PRIMARY> db.mycollection.find({propOne:869}).explain()                                                                 
{
	"cursor" : "BtreeCursor propOne_1_propTwo_1_propThree_1",
	"nscanned" : 8889642,
	"nscannedObjects" : 8889642,
	"n" : 804382,
	"millis" : 11683,
	"nYields" : 152,
	"nChunkSkips" : 0,
	"isMultiKey" : true,
	"indexOnly" : false,
	"indexBounds" : {
		"propOne" : [
			[
				869,
				869
			]
		],
		"propTwo" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		],
		"propThree" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		]
	}
}
mydb:PRIMARY> db.mycollection.find({propOne:869, propTwo:"my string name"}).explain()                                        
{
	"cursor" : "BtreeCursor propOne_1_propTwo_1_propThree_1",
	"nscanned" : 232225,
	"nscannedObjects" : 232225,
	"n" : 232225,
	"millis" : 381,
	"nYields" : 4,
	"nChunkSkips" : 0,
	"isMultiKey" : true,
	"indexOnly" : false,
	"indexBounds" : {
		"propOne" : [
			[
				869,
				869
			]
		],
		"propTwo" : [
			[
				"my string name",
				"my string name"
			]
		],
		"propThree" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		]
	}
}