Every script is written by someone else and might use different values. Mostly they use different difficulties. Some calculators have formulas to project future rise of difficulty and some dont, so you will always get different values. Which is the most trustworthy i cannot tell you.

Here is one with increasing difficulty formula: http://www.mycryptobuddy.com/EthereumMiningCalculator

I tend to use the most pessimistic forecast and then im happy if i get more, but if prices keep falling i will also have to look for a more accurate solution