Even simpler; the underlying securities in the PTTRX portfolio increased in value and when divided by all the outstanding shareholders the increased value of the portfolio resulted in a higher NAV. The PTTRX NAV is not a derivitive soley on stock market trades it is also based on the value of what it owns in its portfolio.
It's as if 1000 people bought a house worth $10,000 an each shareholder put in $10. Then the house went up in value to say $12,000. If that happened each shareholder's basis in the portfolio would be $12 compared to their original investment of $10.
Now add the change in portfolio NAV to the fact that shares in the house can be traded on the open market, there will also be fluctuations in NAV due to market demand in ownership of the house. This is why it takes several hours for Pimco to compile the value of its entire portfolio each day for posting by 3:00 pm PCT since ther are hundreds of thousand of diferent "houses" held in the PTTRX portfolio. I'd like to know how they keep track of it all and can issue a NAV valuation two hours after market close but they do it accurately each and every day.