[graph-tool] Edge betweenness centrality

Hi, I'm a bit confused with the way edge betweenness is being computed by graph-tool. The following .dot file gives me an edge betweenness of 2 for edges 1--3 and 2--3, but edge betweenness should be normalized and therefore less than 1... any ideas? Graph G { 1; 2; 3; 1 -- 2; 2 - 3; } Cheers, Rui