Matching imposes a tree structure on the matched subgraph that corresponds to the model's structure. It is this match tree that is displayed by AVE in the figure.
Vertical compression is achieved by pruning subtrees below each p-chain.
This is as expected. In the first step of the compression, each process handles 32 leaf nodes of the image, performing 16 compression operations. In the last step, only one process is active as evidenced by the 1,1 label on its node. So far, our output was as expected. We see a problem.