The {$natural:1} results are correct

> db.foo.find({}).sort({$natural:-1}).itcount()
500463
> db.foo.find({}).sort({$natural:1}).itcount()
647227
> db.foo.find({}).sort({$natural:-1}).explain()
{
"cursor" : "ReverseCursor",
"nscanned" : 500463,
"nscannedObjects" : 500463,
"n" : 500463,
"millis" : 151,
"indexBounds" : {

}
}
> db.foo.find({}).sort({$natural:1}).explain()
{
"cursor" : "BasicCursor",
"nscanned" : 647227,
"nscannedObjects" : 647227,
"n" : 647227,
"millis" : 194,
"indexBounds" : {

}
}