Measuring .7 volts on pin 2 and 0.0 on the rest when setting all pins low

Question

Having my Arduino Nano 328P disconnected from any circuitry and setting all pins to pinMode(OUTPUT) and writing LOW to (almost) all the pins, when I connect my multimeter to every output pin and the Arduino ground, I measure 0.7 volts on pin 2 and 0.0 volts on 3 to 19. Here I mean the pin numbers on the edge of the Nano, not the processor pins.

For my project it worked out fine because I needed those 0.0 and not the 0.7, but I'm afraid it suddenly might not be pin 2 that's 0.7 in the future and my circuit would fail.

Why is there this one pin giving 0.7 instead of 0.0 and how can I make sure or argue that it will only be pin 2 that gives 0.7?

EDIT as per request, the full test code is below.

void setup() {
  for (int i=1;i<20;i++){
  pinMode(i, OUTPUT);
  }
}

void loop() {
  for (int i=1;i<20;i++){
    digitalWrite(i, HIGH);  
  }
    delay(3000);               
    for (int i=1;i<20;i++){
    digitalWrite(i, LOW);   
    }
    delay(3000);               
}

Answer 1

For future reference, thanks to Majenko: these values fall within the expected voltage output range of 0-0.9V, and you cannot be certain it will stay this way. See the datasheet page 313.