It's a rough average and as some have said, there's a safety factor built in. Wheels designed for, say, a 180 lb rider will see momentary loads far exceeding that due to pavement irregularities. cornering loads, especially on favorably crowned road, etc. The wheels don't just fold up if slightly overloaded, but a wheel pushed to its limits regularly should never be expected to be as durable as something built with more reserve strength.