Strange nextObject behavior - only a bunch of documents

I wrote a simple mapping-script for a Database with around 5M of documents. The find-statement finds all documents when i run it standalone (so it can't be that). The script also ends as expected, no error message thrown. But here's the point - it ends to soon. After about 200k documents and my question is: why?

function performMapping (connectionInputDB, connectionOutputDB) {
    var collectionInput = connectionInputDB.collection(config.input.collection),
        collectionOutput = connectionOutputDB.collection(config.output.collection),

        successCounter = 0,
        errorCounter = 0,

        cursor = collectionInput.find({ 'changed': { '$gte': config.min } }),

        onNextObject = function (error, obj) {
            var mappedObject;

            if(error) {
                console.log(error);
                process.exit(1);
                return;
            }

            if(obj === null) {
                console.log('Success:', successCounter);
                console.log('Error:', errorCounter);
                console.log('Time:', (new Date().getTime() - startTime) / 1000);
                process.exit(0);
                return;
            }

            try {
                mappedObject = mapScript(obj);
                collectionOutput.update(
                    { 'meta.identkey': mappedObject.meta.identkey },
                    mappedObject, 
                    { upsert: true }, 
                    function (error) {
                        if(error) {
                            console.log('Error:', error);
                            errorCounter += 1;
                            return;
                        }

                        successCounter += 1;

                        cursor.nextObject(onNextObject);
                    }
                );

            } catch(ex) {
                console.log('Error:', obj.data.identkey, ex);
                errorCounter += 1;
                cursor.nextObject(onNextObject);
            }
        };

    cursor.nextObject(onNextObject);
}

I think nextObject doesn't work as expected. And the strangest part of it: its always a different amount of successful inserts/updates.