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.