If you draw a straight line on a white board and ask people how long the line is, different people will have different answers.
However if you ask the same folks around how long the line B is in comparison to line A, almost unanimously you’ll get the same reply, i.e. “double the size of line A”
The reason – we humans are visual people. We struggle quantifying something in absolute terms but it’s not that difficult to compare one quantity with another.