Dendrogram heights and topology carry the meaning; leaf order on the page does not.
highlighted = computed this step
Lower joins mean more similar
A node's height tells how far apart its members are. W and X join lowest, at height 1, so they are the most similar pair in this tree.
h(W,X)=1
Leaf order is not distance
Z joins last, at height 3, so it is most distant from the rest. The left-to-right leaf order is just a drawing choice; heights and topology carry the meaning.