The “Ernie Els” document was entered manually via the Atlas web UI. It has an actual ObjectId for the comp sub-document.
When I use my PHP site to enter new records (still under development, obviously…), the format appears as a String, which is the 2nd document with my playerName.
I would try: 'id' => ObjectId($_SESSION['compId']),
or 'id' => new MongoId($_SESSION['compId']) /* this might throw an Exception if compId is not a valid reference syntax */
That code was untested however as I only started learning MongoDB and am thinking of changing implementation in my application.