In lighter baking, you're using less of the butter and oil that can hide a host of measurement sins. One cook's "cup of flour" may be another cook's 1¼ cups. Why the discrepancy? Some people scoop their flour out of the canister, essentially packing it down into the measuring cup, or tap the cup on the counter and then top off with more flour. Both practices yield too much flour.
"Lightly spoon flour into dry measuring cups, then level with a knife," advises Test Kitchen Director Vanessa Pruett. A dry measuring cup is one without a spout―a spout makes it difficult to level off the excess flour with the flat side of a knife. "Lightly spoon" means don’t pack it in.