Mongo pull discrepancy

gistfile1.json

> var x = {x: 0, y: [ {a:1,b:100}, {a:2,b:101} ]}
> db.tester.drop()
true
> db.tester.insert(x)
> db.tester.findOne()
{
	"_id" : ObjectId("4fa7803455bdd0772d514ffc"),
	"x" : 0,
	"y" : [
		{
			"a" : 1,
			"b" : 100
		},
		{
			"a" : 2,
			"b" : 101
		}
	]
}
> db.tester.update({}, {$pull: {y: {$elemMatch: {a: 2}}}}, false, true)
> db.tester.findOne()
{
	"_id" : ObjectId("4fa7803455bdd0772d514ffc"),
	"x" : 0,
	"y" : [
		{
			"a" : 1,
			"b" : 100
		},
		{
			"a" : 2,
			"b" : 101
		}
	]
}
> db.tester.update({}, {$pull: {y: {$elemMatch: {a: 2,b:101}}}}, false, true)
> db.tester.findOne()
{
	"_id" : ObjectId("4fa7803455bdd0772d514ffc"),
	"x" : 0,
	"y" : [
		{
			"a" : 1,
			"b" : 100
		}
	]
}