Square books shade popular sides, push parlays and SGPs, run promotional prices, and limit accounts that consistently beat them. Their lines follow the sharp market with a lag - and that lag, plus promos, is exactly where +EV opportunities against them appear. To be fair, retail books beat sharp books at breadth: vastly more prop markets, better apps, and features like same-game parlays that sharp books don't offer.
The anchor-vs-retail comparison is the core loop of the +EV scanner: devig the sharp anchor, then flag retail prices that beat fair value. Retail books' huge prop menus are a big share of ParlayAPI's 4,000+ prop market keys seen per day.