Most units don't have this problem — cup does
A meter is exactly one meter everywhere in the world; a kilogram is exactly one kilogram everywhere. Cooking's "cup" measurement is different — it's a customary volume unit that was standardized separately in different countries, and those standardizations didn't converge on the same number. This is a genuinely unusual situation among common measurement units, and it's the single biggest source of error when converting recipes across regions.
The main cup sizes in circulation
The US customary (legal) cup, used in American recipes and by this site's converters, is defined as exactly 236.588237 ml (derived from the US customary volume-unit system). The metric cup, used in Australian, New Zealand, and many UK recipes, is defined as exactly 250 ml — a simpler, rounder number chosen for metric convenience rather than derived from any US customary unit. A recipe's "1 cup" can mean either value depending entirely on where it was written, with no universal way to tell just from the word "cup" alone.
A worked example showing the error compounds
A recipe calling for 4 cups of liquid means 946.35 ml if it's using the US customary cup, but exactly 1,000 ml if it's using the metric cup — a gap of about 53.65 ml, or roughly 5.4% of the total volume. For a single cup this difference (about 13.4 ml) might not matter much in a forgiving recipe, but it compounds across every "cup" measurement in a multi-ingredient recipe, and matters considerably more for precision baking, where liquid-to-dry ratios directly affect texture.
Why this matters more for baking than for general cooking
Baking is far less forgiving of measurement error than most savory cooking, since it relies on precise chemical ratios (flour to liquid to leavening) to get the right texture and rise. A 5% volume error from cup-size confusion is unlikely to ruin a soup or stew, but it can noticeably affect a cake's crumb or a bread's rise — which is exactly why professional and precision baking recipes increasingly specify ingredients by weight (grams) instead of by volume (cups) in the first place, sidestepping the cup-size ambiguity entirely.
How to convert correctly
First identify which cup size the recipe's country of origin actually uses — American recipes mean the US customary cup; Australian, British, and New Zealand recipes generally mean the 250 ml metric cup. Only then apply the matching conversion factor. When in doubt and the recipe allows it, converting to a weight measurement (grams) for dry ingredients sidesteps the cup-size ambiguity entirely, since a gram is unambiguous everywhere in a way a "cup" simply isn't.