Why are swallowing a pebble and deliberately eating normal food treated separately?

Kitab al-Sawm: Fasting Hours, Invalidators, Qada and Kaffarah

• Every swallowed object is non-invalidating
• Pebble requires expiation while food requires nothing
• A non-nutritive object requires qaḍāʾ, while specified deliberate Ramadan nourishment also requires kaffārah
• A pebble is necessarily nourishing