Why can a factory freezer make smoother ice cream than a slow home refreeze?

Answer: More crystals start tiny

Air bubbles cut heat — Not quite. Air bubbles can slow heat flow and change melting, but they are not the main reason factory freezing beats a slow home refreeze. The key moment is ice formation itself. Fast heat removal creates many tiny crystal nuclei before existing crystals have much time to grow.

More crystals start tiny ✓ — Right. Fast freezing promotes many nucleation events, so the same water is divided among many small crystals instead of fewer large ones. Commercial scraped-surface freezers also scrape ice from a cold wall and disperse it while whipping air in. That is hard to recreate after a pint has melted, because a home freezer mostly hardens the mass without vigorous scraping.

Sugar lowers freezing point — Not quite. Sugar lowering the freezing point is one reason ice cream stays scoopable, but it does not explain the factory-versus-home refreeze gap by itself. A sugary melted pint can still refreeze gritty if cooling is slow. Smoothness depends on making many small crystals quickly and limiting later growth.