Sometimes people think a person doesn't really believe something if they don't live by it. It can be easier to take a person's advocating for some truth about how we ought to live seriously if they follow their own advice. For the same reason, someone not practicing what they preach can be harder to take seriously. People may also regard ethical standards as not possible to meet until they some who actually meets them.

There's a partial truth there, but it's not the whole story.

Knowing something is true and living as if it is true are different. Philosophically understanding something as true doesn't automatically reform the beliefs and habits that shape your daily life while you're not actively self-reflecting on its truth. So a person can in one sense think their ethical theory is true, and yet consistently still fail to live up to it.

People have to navigate the world prior to coming to complex philosophical understandings if they ever even get there, and this is part of why shaping good habits plays a major role in pre-philosophical educations in some philosopher's political works. It's better to later recognize why your habits are good, than learn the hard way that your habits are bad and then have to work at replacing them with better ones.