The P in the numerator stands for price, or stock price, which is determined by stock market investors. Stock market investors are very quick to react to the price of gold going up, and may buy gold stocks indiscriminately to catch the trend. P represents the equity value of the business based on stock market investors.
NAV stands for Net Asset Value, which is like doing a DCF on each mine separately (as different geographies / mines may have different risk profiles and discount rates), combining the present values, and subtracting out the headquarter corporate costs and any liabilities. NAV represents the intrinsic equity value of the business based on modeling out the cash flows.
NAV is determined by equity research analysts, who would be more cautious than stock investors and slower to react. Updating their models for a rising gold price takes time, and analysts also do want to appear inconsistent by constantly changing their recommendations. As a result, in a rising gold environment, price often moves up faster than NAV, so P/NAV ends up being over 1.