I'm creating a collection with 1000 random points roughly in the area where Canada would be on the globe:

for (i=0; i<1000; i++) {
   var lon = -25 - Math.random() * 75;
   var lat = 35 + Math.random() * 35;
   db.data.insert({"loc": { "type" : "Point",
                             "coordinates" : [ lon, lat ] } });
}

In other words, longitude is between -100 and -25, and latitude is between 35 and 70. I'm defining a 2dsphere index on it:

db.data.ensureIndex({"loc":"2dsphere"})

and then query with a bounding rectange that should encompass all the points:

db.data.find({"loc":
  {"$geoWithin":
    {"$geometry":{"type":"Polygon",
                  "coordinates":[[[-110, 30], [-110, 75], [-20, 75], [-20, 30], [-110, 30]]]}}}}
)

(The polygon runs from -110 to -20 longitude and 30 to 75 latitude.) However, the query does not return all 1000 documents (in one particular example, I only got 919). If I make the rectangle even larger, it actually returns even less documents. This can be confirmed both by .count() and .toArray().length.

Is there anything wrong in my setup or assumptions how $geoWithin works?