Xamarin/C#/GPS: Weird distance measurement

Question

I'm having a difficult time figuring out how to properly get distances between two Locations in Xamarin. At least I'm consistently getting the wrong results, according to almighty Google.

This code explains the problem.

    public void OnLocationChanged(Location location)
    {
        // Longitude/Latitude of Tower Bridge.
        location.Longitude = 51.5053446;
        location.Latitude = -0.0765396;

        foreach (var store in this.stores)
        {
            if (store.DistanceView != null)
            {
                Location store_loc = new Location(location.Provider);
                //store_loc.Longitude = double.Parse(store.GPSN);
                //store_loc.Latitude = double.Parse(store.GPSW);

                // Longitude/Latitude of Big Ben.
                store_loc.Longitude = 51.5005747;
                store_loc.Latitude = -0.1247025;

                var distance = location.DistanceTo(store_loc);

                // Google Maps ("measure distance") says 3.44km.
                // Xamarin (variable "distance") says 5351.983 meters.

                store.DistanceView.Text = distance.ToString();
            }
        }
    }

The actual locations I'm working with are different (for reasons of privacy), but the measurement error is similar, in that I'm getting a result that's not quite twice as high as measured by Google Maps, but somewhere in the vicinity. At any rate, the above measurements should match, and they don't.