Here's a breakdown of why:
* The original meaning: The phrase originates from a Latin phrase "exceptio probat regulam in casibus non exceptis". It means, "the exception proves the rule in cases not excepted". The original meaning focused on legal contexts where there was a general rule, and exceptions were specifically listed. These exceptions actually confirmed the rule, showing it applied in all other instances.
* The common misinterpretation: Most people interpret the phrase to mean that if there's an exception, it validates the rule. This is incorrect. An exception actually challenges the rule and suggests it might not be universally applicable.
* A better phrasing: To avoid the misleading implication, it's better to use a different phrase like: "Exceptions demonstrate the limits of a rule."
In short, the phrase "the exception proves the rule" is often used incorrectly. It is better to focus on the idea that exceptions highlight the limitations of rules, rather than validate them.