The ruler metric itself is also a factor. And the human looking at it.

If you were using cm , thats going to give you a different number than if you were using mm. Or nm.

Humans can't perceive nm by eye. And most can't distinguish more than 1/3 or 1/4 of a mm. At best.

So the tool and the human also limits accuracy. Meaning the tool has influenced the result. And The human has limited the measurement too.