How to solve the issue of "%" on Calculator giving different results on iPhone vs Mac?
Here's a short article that does a good job of describing the behaviour you're observing with OS X's in-built Calculator app: How does the calculator percent key work?

What's going on?

When performing an operation involving two values followed by the percent key, the product is first calculated (i.e. 2000 x 19 % = 380) and this then replaces the second value in the ongoing calculation, which becomes 2000 x 380. When you then press the division key ÷ the product of the ongoing operation is calculated (2000 x 380 = 760,000), as you described, before you input the divisor to perform another operation.

Working with percentages using OS X's in-built Calculator

To work with percentages in this instance you should express them in decimal form:

2000 × 0.19 ÷ 12
= 31.6666


This works with both iOS's and OS X's Calculator. I would advise against using the percent key in favour of working with decimals. The same can be said when using physical calculators, where the behaviour can differ between models.