I installed Pipelight from the PPA archive, according to these instructions.
The installation command,
sudo apt-get install --install-recommends pipelight-multi was for only one package, but it entailed downloading and installing about 180 dependency packages, about 40 of which were upgrades to currently-installed packages.
Therefore, about 140 new packages were installed.
Pipelight subsequently didn't work, so I removed it:
sudo apt-get purge pipelight-multi
No other packages were removed.
I then used
sudo apt-get autoremove. This command removed all auto-removeable packages. There were exactly 100 of these (there were also exactly 100 packages in Synaptic Package Manager's 'Autoremovable' list).
So this left about 40 packages that weren't considered auto-removeable, and so weren't removed. I had to remove them manually.
There were no unmet dependencies on my system before I installed Pipelight; and I hadn't marked any packages as manually-installed, installed or removed anything else, or anything like that. Only about ten minutes passed between installing Pipelight and removing it.
So after I removed Pipelight, why weren't all of its unneeded dependencies marked as auto-removable?