The sample diagram you posted is structurally not a tree. For this reason, the tree layout algorithm ignores offending (i.e. non-tree) edges when arranging nodes. In your case, the edge connected nodes 6 and 7 is ignored (and node 7 is positioned as if that edge would not exist).
As an alternative, try the hierarchical layout algorithm with option "Symmetric Placement" in tab "General" checked.