Glossary
Compressed memory
Memory macOS has squeezed in place to make room, before it writes anything to disk.
Before macOS writes memory to swap, it compresses memory that has not been used recently and keeps it in RAM. Activity Monitor shows the total as Compressed. Compression costs some processor time each time the data is needed again, but far less than reading it back from disk.
Compressed memory still belongs to the app that owns it. That is why Sentinely estimates with each process's memory footprint, which includes its compressed memory, instead of resident size, which leaves it out.