Odd spacing behaviour

Sorry for the bland subject, but I found it hard to formulate ...anyway try
this command:

  echo ".pl 1\nShall I compare thee to a summer's day?\n\l'16.5c'" | groff -T ascii
  
and you'll see the output

  Shall     I     compare     thee     to     a    summer's    day?
  _________________________________________________________________
   

Why are the words spaced out to fill the length of the line? Of course there
is no break between the text and the line and the line then cannot fit
within the length of the line, but this behaviour seems weird. Is that
expected behaviour?

Regards,
Morten