Why is net profit margin always lower than gross profit margin?

Yes, the net profit margin is always lower than the gross profit margin. It's because the gross profit margin is equal to the cost of goods sold subtracted from revenue, then divided by revenue. While the net profit margin is determined by subtracting revenue from the cost of goods sold, operating expenses, and other expenses divided by revenue.

To make sure that your calculations are right, make sure that you work out your expenses accurately and that your pricing is correct, as these are common pitfalls when calculating profit margins.