Depends on the diff, many times a site told me i would get like 10k coins in 24 hours, at the end it was 15k or 1k or 4k.. Depends if the diff is getting higher or lower.
You are using calculator wrong. It shows rewards for the
past 24 hours.
Difficulty defines amount of hashes required to mine a block in target time, depending on your real number of submitted hashes can be lower or higher than the expected value.
But luck is always close to 100% on larger intervals, so it's possible to accurately calculate rewards based on historical data.
Yeah maybe they can do it when all the miner tools are ready and released. Those new mining tools gives u big results, so at this moment a calculator will be 100% incorrect.
Also about the results in the past, yeah its like u say the past. Nobody want to know what they could mine yesterday, they want to know how many they mine right now. If the diff was low yesterday the calculate will be totally wrong when the diff is 10x higher today, also if the diff over 2 days is 30x higher.
Those calculators are fun for beginners but most of the time they are simply not correct. Maybe it work on a coin like Ethereum what already have a sort of a stable difficulty, but new coins nah, they are most of the time incorrect.
So personally i dont think this mining calculator must be top priority, i agree (and i guess the devs will do) its a funny thing to have for newcomming people in the mining world.