bug#18946: a problem or a 'bug' about the command groups

[yangzhao]

Excuse me,
             I use command 'group -d ' to delete   user  ak from a group  
testgroup. Then ,something confusing comes to me.I use command groups as ak 
,and get ak ,testgroup  ,them two. But when I use groups ak , I get ak only. 
And I take a look at /etc/group for testgroup ,there is nothing with ak,so I 
want to ask ,which file does command groups look up ?
                                                                                
                                                                                
                                                   Thanks,   
                                                                                
                                                                                
                                                Yours,sincerely