In the real world, you’re measuring with significant figures.

You draw a 1 cm line with a ruler. But it’s not really 1 cm. It’s 0.9998 cm, or 1.0001, or whatever. The accuracy will get better if you have a better ruler: if it goes down to mm you’ll be way more accurate than if you only measure in cm, and even better if you have a nm ruler and magnification to see where the lines are.

When you go to measure the hypotenuse, the math answer for a unit 1 side triangle is 1.414213562373095… . However, your ruler can’t measure that far. It might measure 1.4 cm, or 1.41, or maybe even 1.414, but you’d need a ruler with infinite resolution to get the math answer.

Let’s say your ruler can measure millimeters. You’d measure your sides as 1.000 cm, 1.000 cm, and 1.414 cm (the last digit is the visual estimate beyond the mm scoring.) Because that’s the best your ruler can measure in the real world.