User created with different UID and GID

I'm creating a new user in my system, by calling:

sudo useradd -m -G nas -s /bin/bash alf

which supposedly should create a new user with the same UID and GID. However the passwd file is appended with the line:

alf:x:1004:1007::/home/alf:/bin/bash

Which has 1004 for UID and 1007 for GID. Other users I've created have the same UID and GID, so why is this happening?