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
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.