Does overwrite_params ignore routing priority?

My routes.rb looks like this:
map.resources :programs, :member => { :arrivals => :get }
map.connect ':controller/:action/:id'
map.connect ':controller/:action/:id.:format'

Running 'rake routes' (omitting some of the routes for brevity):
arrivals_program GET /programs/:id/
arrivals
{:controller=>"programs", :action=>"arrivals"}
formatted_arrivals_program GET /programs/:id/
arrivals.:format
{:controller=>"programs", :action=>"arrivals"}
                                    /:controller/:action/:id
                                    /:controller/:action/:id.:format

Now let’s say I’m at this url: ‘http://myserver/programs/2/arrivals’.
My view has this code:
<%= link_to ‘Export to XML’, :overwrite_params => { :format => :xml }
%>

This produces the link: http://myserver/programs/arrivals/2.xml. I
expect it to produce http://myserver/programs/2/arrivals.xml.

For some reason it picks the lowest priority route,
the /:controller/:action/:id.:format route at the bottom of routes.rb.

When I delete /:controller/:action/:id.:format from routes.rb, it
correctly spits out http://myserver/programs/2/arrivals.xml.

Anyone know what's going on here?