This happens because the fitBounds() needs to snap to a viewport that fits the map canvas using the maximum zoom level possible. On the other hand, the viewport returned by the geocoder is not dependent on the size, layout or zoom level of the map canvas. Therefore the fitBounds() method adjusts the map's viewport in order to view the passed LatLngBounds in full at the centre of the map.
You may want to check the following example for a clearer demonstation:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
<title>Google Maps fitBounds Demo</title>
<script src="http://maps.google.com/maps/api/js?sensor=false"
type="text/javascript"></script>
</head>
<body>
<div id="map" style="width: 500px; height: 350px"></div>
<script type="text/javascript">
var myOptions = { mapTypeId: google.maps.MapTypeId.ROADMAP };
var map = new google.maps.Map(document.getElementById("map"), myOptions);
var geocoder = new google.maps.Geocoder();
geocoder.geocode({'address': 'RU'}, function (results, status) {
var ne = results[0].geometry.viewport.getNorthEast();
var sw = results[0].geometry.viewport.getSouthWest();
map.fitBounds(results[0].geometry.viewport);
var boundingBoxPoints = [
ne, new google.maps.LatLng(ne.lat(), sw.lng()),
sw, new google.maps.LatLng(sw.lat(), ne.lng()), ne
];
var boundingBox = new google.maps.Polyline({
path: boundingBoxPoints,
strokeColor: '#FF0000',
strokeOpacity: 1.0,
strokeWeight: 2
});
boundingBox.setMap(map);
});
</script>
</body>
</html>
Below are some screenshots for various countries from the above example. The red bounding box is the viewport returned by the geocoder. Note the difference between the bounding box and the actual viewport, as adjusted by fitBounds():
Country: US
Country: RU
Country: IT
