[graphviz-interest] A problem with too many edges

Consider the attached y.gv.

What is it exactly which causes an edge to be drawn from node_1 to node_3?

I've read the 'Subgraphs and Clusters' section of 
http://www.graphviz.org/content/dot-language but AFAICT nothing there 
tells me an edge pointing to a subgraph is necessarily explicitly 
connected to every node in that subgraph.

Am I missing something?

-- 
Ron Savage - savage.net.au

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strict digraph
{
	node_1
	->
	{
		node_2
		->
		node_3
	}
}