My guess would be on the days you work, your body can stay in a working mode/mindset, whereas when you try to do so on non-work days, you have to force yourself into that mindset, which requires a lot of self control. It also might be your body telling you to take a break, and you're overriding that feeling by writing.
You have ADHD? Because this is a pretty common thing for people with ADHD. Resting opens you up to getting locked in by executive dysfunction, but keeping busy and being able to bounce between things that you’re motivated to do lets you hyperfocus through a bunch of shit.
Because your brain only uses words for a small fraction of its processing, but the things being processed span across both parts of your brain. As you work with your body, your brain is doing math and gaining insight about the world. It’s helping to process information involved in the things you write about.
Non-verbal activities are an excellent way to keep working on problems you originally conceived verbally, when it’s time for that part of you to take a break.
And just basic interaction with the concrete world is in some ways like consulting a computerized oracle that, while speaking in a strange tongue, never says anything untrue.
Also exercise gets your blood moving. Best way to get your brain well-supplied with oxygen, nutrients, and growth hormones is to exercise. It’s as good as coffee.
I have a couple of ideas. One is that variety promotes creativity. The other is that you might be thinking more deeply about your writing while you are away, but while you are forcing yourself to do it you are pushing it away.