I too had the same issue when flying my Phantom 4 the other day, and was also around the 42% mark. I remember thinking just as you that it was odd to be THAT low on voltage, but still have 42%ish remaining / reported. To take note, I slowed down, hovered and it jumped back up... I was in sport mode being crazy (crazy AND responsible lol) so, as with any UAV i've piloted, as well as my custom builds, when you juice the throttle it puts a decent drain on the cells. This is typically, in my experience, due to a lower "C" rating of the batteries. I wonder what the DJI smart batteries are rated at C wise... Now I will have to find that out!
Side note though, I stayed in the air, and flew more "normal" and not full tilt, and all was well until I landed at around 18%