FQL: A friend's specific uid DOESN'T show on User table

Question

1. I get all my friend's uids (for all those friends who grant permission):

   >>> my_function_fql('SELECT uid2 FROM friend WHERE uid1=me()')
   

2. Among all those uids, there's one which seems not to exist in User table:

   >>> my_function_fql('SELECT name FROM user WHERE uid=%s' % ghost_uid)
   []
   >>> my_function_fql('SELECT uid2 FROM friend WHERE uid1=me() AND uid2=%s' % ghost_uid)
   1234567890
   

In fact, I've found what contact it is. Nothing unusual about her account is noticeable. =/

Why does this happen?