MongoDB find, limit skipped some documents

Question

Using Mongoose for MongoDB query. I have inserted 100 records using code below:

console.log("begin inserting posts...");
let i;
for (i = 0; i < 100; i++) {
    const post = new Post({
        author: {
            name: "name" + i
        },
        source: "Qizhongyouxiang",
        url: "https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg",
        title: "image title"
    });
    post.save();
}

Since the insertion is in sequence, so the first five records should be with name from name0 to name4.

This is how the data look like through Compass:

{"_id":{"$oid":"5dfc4386006c085bda8916db"},"author":{"name":"name0"},"source":"Qizhongyouxiang","url":"https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg","title":"image title","__v":{"$numberInt":"0"}}

{"_id":{"$oid":"5dfc4386006c085bda8916dc"},"author":{"name":"name1"},"source":"Qizhongyouxiang","url":"https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg","title":"image title","__v":{"$numberInt":"0"}}

{"_id":{"$oid":"5dfc4386006c085bda8916dd"},"author":{"name":"name2"},"source":"Qizhongyouxiang","url":"https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg","title":"image title","__v":{"$numberInt":"0"}}

{"_id":{"$oid":"5dfc4386006c085bda8916df"},"author":{"name":"name4"},"source":"Qizhongyouxiang","url":"https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg","title":"image title","__v":{"$numberInt":"0"}}

{"_id":{"$oid":"5dfc4386006c085bda8916e0"},"author":{"name":"name5"},"source":"Qizhongyouxiang","url":"https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg","title":"image title","__v":{"$numberInt":"0"}}

Then I use the following code to retrieve 3 records:

        Post.find().limit(sizeLimit).then(posts => {
            console.log(posts.length);
            console.log(posts);
            res.json(posts);
        });

The output is like this:

3
[
  {
    author: { name: 'name0' },
    _id: 5dfc4386006c085bda8916db,
    source: 'Qizhongyouxiang',
    url: 'https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg',
    title: 'image title',
    __v: 0
  },
  {
    author: { name: 'name1' },
    _id: 5dfc4386006c085bda8916dc,
    source: 'Qizhongyouxiang',
    url: 'https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg',
    title: 'image title',
    __v: 0
  },
  {
    author: { name: 'name4' },
    _id: 5dfc4386006c085bda8916df,
    source: 'Qizhongyouxiang',
    url: 'https://media.wired.com/photos/5b899992404e112d2df1e94e/master/pass/trash2-01.jpg',
    title: 'image title',
    __v: 0
  }
]

Any reason the records with name2 and name3 got skipped? If I set the limit to 5, name2 and name3 record will show after name4.