auto-fill breaks line too early

[mandar.mitra]

I type the following (single) line in Emacs 24 [1] (started as emacs -Q):

--------------------
A study of the number of relevant documents for queries may provide some 
insights about the collections when juxtaposed across the FIRE years. Voorhees 
and 
--------------------

I turn on auto-fill-mode, place cursor on i (for insight) and hit C-x f to
set fill column to 73. Then I go to the end of the line and hit space. This
produces:

--------------------
A study of the number of relevant documents for queries may provide some
insights about the collections when juxtaposed across the FIRE
years. Voorhees and  
--------------------

Why does auto-fill break the line before years instead of after? This
happens across emacs versions, whenever there is a sentence break close to
the fill-column. If I delete the period, the line break is inserted after
years as expected.

Is this a bug or a feature? Is there a workaround or should I file a bug /
look in the code?

Many thanks,
Mandar.



[1] GNU Emacs 24.1.50.1 (i686-pc-linux-gnu, GTK+ Version 3.2.0) of 2012-09-19 
on litembilla, modified by Debian