Best fit vs Full library in the Templates panel

Click the layout fit button next to Filter at the top of the Templates panel, or press L to switch. Best fit (on by default) lists the layouts that suit your photos' aspect ratios, best matches first, mixing justified layouts with the built-in templates that crop your photos the least - and photos you drop onto a spread go into one of the top matches. Full library shows every template for the photo count, with that spread's justified layouts after them, and works the way the panel always has.

The Layout fit menu open at the top of the Templates panel, with Best fit checked above Full library and a description of each