Why some entries of last -a do not show the hostname ?

Issue

• The command last -a does not show a hostname on all lines as it should:

   -a     Display the hostname in the last column. Useful in combination with the next flag.
  

• The result is something like:

  root     pts/3        Fri Feb 28 04:15   still logged in
  root     pts/2        Fri Feb 28 04:09   still logged in    
  root     pts/1        Fri Feb 21 04:44   still logged in    user1.serv
  root     pts/0        Fri Feb 21 04:44   still logged in    user2.serv