req.session.id different to req.session._id

I would like to understand why this:

req.session._id = doc._id;
req.session.id = doc._id;

console.log(typeof req.session._id); // object
console.log(typeof req.session.id); // string

console.log(req.session._id); // 4oqTbA06DcK9cIiU3tnK62Ic.0puZ9uPnIaghMBIQKaIbhIHYzeP8wtS3MXnzpi+yC1c
console.log(req.session.id); // 4fb01ad2ca42e9552d000001

So, when i want to do this:

var myID = req.session._id, myIDm = new BSON.ObjectID(myID); // don't bug
var myID = req.session.id, myIDm = new BSON.ObjectID(myID); // bug: Argument passed in must be a single String of 12 bytes or a string of 24 hex characters

Thank you for tell me all information about this!