:index option for form select

I've found the :index option to be useful and necessary for form
methods when I have a form that contains fields for multiple records,
some of which have already been saved previously and some of which are
new.

However I ran into a problem today when I was trying to do this with a
form that had a pull down select field for each of several records -
the :index just wasn't being inserted into the params hash. After much
frustration and investigation I finally found the explanation in the
api documentation (http://api.rubyonrails.org/classes/ActionView/
Helpers/FormOptionsHelper.html) -

"Like the other form helpers, select can accept an :index option to
manually set the ID used in the resulting output. Unlike other
helpers, select expects this option to be in the html_options
parameter. "

This explains exactly why I was having trouble and the problem is now
fixed. But it leaves me with a big question... why??? Why on earth
would they put this option in a different method parameter? Is there
some logic behind this?