Why can cold-brew coffee taste different from hot coffee served over ice?

Answer: Cold water and a longer steep extract a different mix from the grounds.

Cold water and a longer steep extract a different mix from the grounds. ✓ — Brewing temperature and contact time affect which compounds dissolve and how quickly they leave the grounds. Cold brew steeps in cool water for hours, while iced hot coffee was brewed hot before cooling. Serving both cold does not make their composition identical, though beans, grind and recipe also matter.

Ice removes caffeine from hot coffee as the drink cools. — Ice cools and dilutes coffee rather than selectively removing caffeine. When it melts, it adds water, so each sip may contain less caffeine without any being filtered out. Caffeine content also depends on the coffee dose and brewing recipe.

An overnight steep creates new sugar in the coffee grounds. — An overnight steep extracts compounds already in the grounds; it does not normally manufacture new sugar. A sweeter impression can come from the balance of aroma, bitterness and sourness. More soaking time changes extraction, so taste alone does not show that sugar has been created.