Scenario:
I'm currently not night current. But I go flying tonight with Joe. Joe isn't a CFI, but Joe is night current, so he acts as PIC and I log some night PIC time and one takeoff and one landing for the flight. Next week I do something stupid, and the FAA asks to see my logbook. I present them my logbook. Inspector goes through book, and says "Hey, this wasn't a solo flight, and you weren't passenger current. Many of your other flights prior to this indicate solo, looks like you had passengers." (I do actually log solo flights as solo - always have). Inspector decides to ding me with a 61.57, because nothing in the book differentiates it as acting vs logging.
Or even if I didn't go flying with Joe and I was solo (and forgot to log it as solo), still looks like, based on evidence I provided, I had passengers.