mongoose exec call with async.parallel

I have a code like this in my express controller

   function (req, res) {
      var queries = [];

      data.forEach(function (item) {
        var query = myModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true}).exec;

        queries.push(query);
      });

      async.parallel(queries, function (err, docs) {
        res.json(docs);
      });
    });

If data array has 3 item, then i have an array of 3 null values.

async.parallel function accepts a function with a callback argument, that should be called to properly complete its execution. So mongoose.Query.exec does the same. But i recieve an array of null objects as a result.

If i wrap my exec call like so

var query = function (cb) {
  tournamentsModel.findOneAndUpdate({remoteId: item.id}, item, {upsert: true}).exec(function (err, model) {
    cb(err, model);
  });
};
queries.push(query);

everything is ok and i recieve 3 docs from mongo as a result. Why should i explicitly call a callback passed to a async.parallel function call, when exec method does the same?