Question about the opLog for mongodb.

I am using oplog to trigger events based on the database updates.
note: I am reading opLog from node.js, so i'm not sure if this is a node driver issue or just a standard opLog issue.

If you try:

db.notes.insert(

{status:'new',subject:'subject 1'}

)

db.notes.update({_id: ObjectId("512231109946135b79af6bec")},

{status:'pending',subject:'subject 2'}

);

//We expect that both fields are shown in oplog (CORRECT)
oplog ..
{ ts:

{ _bsontype: 'Timestamp', low_: 1, high_: 1361195351 }

,
ns: 'spotmasterNotes.notes',
o2:

{ _id: 512231109946135b79af6bec }

,
op: 'u',
h:

{ _bsontype: 'Long', low_: 1743521552, high_: 1346864790 }

,
o:

{ status: 'pending', subject: 'subject 2' }

,
v: 2 }

db.notes.update({_id: ObjectId("512231109946135b79af6bec")},{$set:{status:'close',subject:'subject 3'}});

//We expect that both fields are shown in oplog (WRONG only one shown)
oplog ..
{ ts:

{ _bsontype: 'Timestamp', low_: 1, high_: 1361195394 }

,
ns: 'spotmasterNotes.notes',
o2:

{ _id: 512231109946135b79af6bec }

,
op: 'u',
h:

{ _bsontype: 'Long', low_: 172060737, high_: -1048766452 }

,
o: { '$set':

{ subject: 'subject 3' }

},
v: 2 }

Any idea why this happens ?