Measuring extents of turf.buffer in Leaflet

When 'kilometers' is specified as the distance option, turf.buffer appears to first calculate a distance in degrees and then create a buffer in degrees rather than km. Thus, the real E-W dimension of a buffer will get narrower as latitude increases.

HTML

<link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.css">
<script src="http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js"></script>
<script src="//api.tiles.mapbox.com/mapbox.js/plugins/turf/v2.0.0/turf.min.js"></script>
<div id="map"></div>

CSS

#map {
    height: 500px;
}

JavaScript

var map = L.map("map").setView([61.441767, 5.470247], 3);

L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

var pt = {
    "type": "Feature",
        "properties": {},
        "geometry": {
        "type": "Point",
            "coordinates": [5.470247, 51.441767]
    }
};


var buffered = turf.buffer(pt, 500, 'kilometers');

L.geoJson(pt).addTo(map);
bufferLayer = L.geoJson(buffered).addTo(map);
var point1=
testScales(pt, bufferLayer);

var pt2 = {
    "type": "Feature",
        "properties": {},
        "geometry": {
        "type": "Point",
            "coordinates": [5.470247, 71.441767]
    }
};

var buffered2 = turf.buffer(pt2, 500, 'kilometers');

L.geoJson(pt2).addTo(map);
var bufferLayer2 = L.geoJson(buffered2).addTo(map);

testScales(pt2, bufferLayer2);

////////////////////////////////////////////////////////////////////////////////////////////
//functions to measure extent of buffer polygons//
//
//When 'kilometers' is specified as the distance option, turf.buffer appears to first
//calculate a distance in degrees and then just create a buffer in degrees rather than km.
//Thus, the real E-W dimension of a buffer will get narrower as latitude increases.
////////////////////////////////////////////////////////////////////////////////////////////

//input: GeoJson point and Leaflet layer for corresponding buffer
//creates line segments on axes, with popups for distance
function testScales(cenPt, bufLayer) {
    var coords = [];
    coords.north = [cenPt.geometry.coordinates[0], bufLayer.getBounds()._northEast.lat],
    coords.east = [bufLayer.getBounds()._northEast.lng, cenPt.geometry.coordinates[1]],
    coords.south = [cenPt.geometry.coordinates[0], bufLayer.getBounds()._southWest.lat],
    coords.west = [bufLayer.getBounds()._southWest.lng, cenPt.geometry.coordinates[1]];
    var scaleLines = {
        type: "FeatureCollection",
       ...