In an interview, the coding challenge is often to produce something new from scratch while being closely monitored by people you don't know, who control your financial future.

When working with a "junior," you'd already be fairly familiar with the code base, build system, and best practices. And with a junior, you're not likely to be solving things that require deep concentration, like never-before-seen problems or architectural work (or screwball interview-tests). And, unlike an interview, if something does require all my focus, it's very easy to defer. Take a break and think about it alone.