They are not writing code without bugs. But the rate of incoming bugs varies from project to project. For mature projects without a lot of churn the bug rate de

They are not writing code without bugs. But the rate of incoming bugs varies from project to project. For mature projects without a lot of churn the bug rate decreases over time. They may also be in a position where they can afford to delay releasing a feature until all bugs are fixed. Most teams on the other hand ship with known bugs, because a feature with bugs today will make more money than a feature without bugs tomorrow.