What single idea unifies PAO, the Ben System, and their relatives?

Pack more items into each image — every one of them trades a bigger, well-drilled system for fewer images and loci per unit of data.

A plain peg holds one item per image; PAO holds three; the Ben System holds even more. The whole family is one design dial — items-per-image — turned up and up. Turning it up costs more upfront (bigger tables, harder drilling) and buys denser, faster storage. Where you set the dial is a personal trade-off between setup effort and raw compression.

Go deeper:

• Ben Pridmore (Wikipedia) — the 'dial turned up' extreme — a 1000-image number system.
• Mnemonic peg system (Wikipedia) — the low end (one item per peg) of the same continuum.