I remember as a kid my parents telling me about a doctor practicing medicine, and I always wondered why they were allowed to do that since obviously “practice” meant they had not mastered it yet. As I got older, I learned that lawyers and musicians also practice their art, but factory workers or even craftsmen like carpenters simply do their job. I think the difference is that to practice a thing means that it will never be the exact same thing twice (even though you practice scales). Even a dentist removing two pair of wisdom teeth from identical twins will find differences. On the other hand, factory workers and carpenters are expected to produce identical results over and over again.
In 1st John 3, verses 4, 7, 8, 9, and 10 John continues his teaching by contrast. The contrast is sin/lawlessness and righteousness. Now you need to check your translations. In most of the modern translations, it says those who PRACTICE sin or PRACTICE righteousness (or some sense of continual persistence). The KJV simply says those who COMMIT sin or DOETH righteousness (which implies once and done). The danger of the KJV is that it sounds like if you screw up just once, you are disqualified for life. Or the flip side danger is that if you do one really big righteous work, then that makes you righteous. Clearly John is talking about a lifestyle of continuous sin or continuous righteousness. Even then, it is not a matter of exceeding some ratio or threshold, and especially not a matter of comparing ourselves with others. Rather it is our propensity for sin versus righteousness.
Consider a pair of clean dice (I just got back from a conference in Las Vegas). They should randomly roll any number from 2-12. John is talking about loaded dice: they can be loaded for sin or righteousness. While they still occasionally roll some odd number, they have a propensity to come up in a given pattern. Our lives are to be changed from loaded for sin to loaded for righteousness. Will we come up with a perfect roll every time? No, but more and more as we practice righteousness, we will, and “In this the children of God are manifest.”