How are they suppose to show luck? It is not like there is a mathematical equation they would apply and tell you ok based on luck and current hashrate next diff is xxxxx.
Why do you assume it's something they don't understand? Chances are they do understand but nothing they can do about it and they expect everyone who sees those numbers understands that there is a missing factor called luck which can effect the final number drastically.
At the moment the one that seems to be preferred to be quoted here all the time is providing ranges ... but they are always wrong