I don't think there is a difference. From Franco-Nevada's website (Gold Royalty Company):

Net Profit Interest ("NPI") or Net Proceeds Royalties ("NPR") are based on the profit realized after deducting costs related to production as set out in the royalty agreement. NPI or NPR payments generally begin after payback of capital costs. Although the royalty holder is not responsible for providing capital, covering operating losses or environmental liabilities, increases in production costs will affect net profit and royalties payable. Link: http://www.franco-nevada.com/assets/types-royalties