If the bags are the same at both sites, the price may appear different at the net-a-porter site if you're outside of the UK and your Net-a-porter site is set to an international location. If your net-a-porter location is set to an international location, VAT won't be displayed in the purchase price. Mulberry charges VAT no matter where in the world you purchase from. This may explain the price difference if the two bags are the same.
In the top left hand corner of the Net-a-porter site, it will usually say Welcome you are in [country here]. If you're actually in the UK and the name of a different country appears, you will be able to fix this by clicking on the correct country name that appears and at the next page.
There may be other reasons for the difference, but this was the first one that came to mind for me.