Hi compare to last year my sales increase from 13%, my gross profit is same 42%, my net profit before the tax increase 37%, so I want to know why its happen?

There are many, many numbers between sales at the top of an income statement, and net profit before tax near the bottom of the income statement. It's impossible to know what happened in there without knowing what every single one of those numbers was. And even then, someone could only tell you mathematically what's going on, and not why it happened. (They could only give you possibilities.)