You also have to be online in order to receive achievements. WHile they technically unlock offline, you don't get notifications for them, nor does your gamerscore increase, until you are online and they connect to Xbox servers.
That's because unlike the 360 (where achievements were completely programmed on to the disc) the XB1 requires a ping on the server in order to count it as "met." So you can unlock hundreds of points, but you won't know it until the next time you connect.