When you edit, you are often creating an deleting segments and nodes, even when you don't realize it.
If you take a moment to select the segments and nodes and look at their IDs in the left pane, you can see what is happening.
Here is what happened during an actual editing session.
Every segment is attached to two nodes - one node at each end of the segment.
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At the junction, two segments are connected to the same node. There is exatly one node at a junction.
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