The wheel differs from each other. Some are steel. Some are cast aluminum. Some are forged aluminum. Some can even be made of carbon fiber (these are obscenely expensive). Because of the materials used and the "pattern" or "style" of the wheel, one wheel can weigh vastly more than another of the same diameter. Heavier wheels are generally considered to be "worse" than lighter ones because lighter wheels reduce something called "unsprung weight." Unsprung weight is the weight that is not supported by the vehicle's springs. While two wheels may have the same diameter — 18 inch alloy rims, in this case — they can have different widths and offsets.