MongoDB - real disk usage is LOWER than dbStats.fileSize

I'm a bit surprised by what I observe in my mongo server, so I would like to know if anybody understand how this is possible.

I have one collection of around 15K documents. For each of them, I store in GridFS 3 files, with an average size of 1MB per file. So the expected storageSize should be something near 60GB (if considering the size of the document itself to be 1MB too). I can't test all my data by hand, but I really want to be sure everything is here and that there is no missing data for some of the documents.

Here is what I get :

> db.stats();
{
    "db" : "myBase",
    "collections" : 6,
    "objects" : 348822,
    "avgObjSize" : 195475.27135329766,
    "dataSize" : 68186075104,
    "storageSize" : 69329817136,
    "numExtents" : 73,
    "indexes" : 14,
    "indexSize" : 42171808,
    "fileSize" : 67108864,
    "nsSizeMB" : 16,
    "dataFileVersion" : {
        "major" : 4,
        "minor" : 5
    },
    "ok" : 1
}

so storageFile ~ 65GB, this is acceptable.

But when I look at the file system

$ ps -edf | grep mongo
user 20728     1  0  2014 ?        05:31:03 /opt/tools/bin/mongod --dbpath=/local/mongo-data

$ cd /local/mongo-data
$ du -h
 4.0K   ./_tmp
 721M   ./journal
 8.8G   .
$ du -h *
 721M   journal
 65M    myBase.0
 2.0G   myBase.37
 2.0G   myBase.5
 2.0G   myBase.7
 2.0G   myBase.8
 17M    myBase.ns
 4.0K   _tmp

How come the data on disk takes LESS space that what it says in mongo ? Why doesn't appear anywhere a stat with a value of 8GB ?

---

I add stats from files and chunks if it can help understand what is happening

> db.fs.files.stats();
{
    "ns" : "myBase.fs.files",
    "count" : 48911,
    "size" : 14316208,
    "avgObjSize" : 292.69914743104823,
    "storageSize" : 18898944,
    "numExtents" : 7,
    "nindexes" : 2,
    "lastExtentSize" : 7647232,
    "paddingFactor" : 1,
    "systemFlags" : 1,
    "userFlags" : 0,
    "totalIndexSize" : 7873488,
    "indexSizes" : {
        "_id_" : 2076704,
        "filename_1_uploadDate_1" : 5796784
    },
    "ok" : 1
}

> db.fs.chunks.stats();
{
    "ns" : "myBase.fs.chunks",
    "count" : 283537,
    "size" : 67787872056,
    "avgObjSize" : 239079.45719958947,
    "storageSize" : 68819078704,
    "numExtents" : 48,
    "nindexes" : 2,
    "lastExtentSize" : 2146426864,
    "paddingFactor" : 1,
    "systemFlags" : 1,
    "userFlags" : 0,
    "totalIndexSize" : 27324192,
    "indexSizes" : {
        "_id_" : 11781616,
        "files_id_1_n_1" : 15542576
    },
    "ok" : 1
}