I have the UNIT NAV for FFN also at $15.70Spreadsheet showing a gain of 39 cents since the close on the 15th of April.
Now i was a little low last month but believe i have corrected the weightings IF that is what the difference was. So many other circumstances can change the NAV, like going to cash to pay distributions, covered call options getting hit, and changes in weightings due to some stocks going up while others tank. As well the managers can reduce position in some of the deemed riskier stock holdings.