That's what defines a right angle. When one line stands against another line, so that the angles on either side of the first line are equal, or "right" to each other. In mathematical terms those angles would have to both be exactly 90 degrees in order to be "equal". Even the slightest difference between them, and they are not considered "right" angles anymore.

This is why the meme above says, "My science teacher: right angles don't exist in nature". Because no naturally occurring structures are exactly 90 degrees. Ever. There is always some tiny variance that breaks that theoretical requirement.

The person I responded to said, "I doubt very many science teachers would have said that", but they do. At least at more advanced levels. It's a common teaching parable that opens the conversation about the inherent "fuzziness" of reality. Even the most accurate measurements will always have a certain amount of baked-in uncertainty.

Reality itself is messy. There are no true right angles. No perfectly parallel lines. No truly flat surfaces. The best you can ever do is get ridiculously close.