Might it be that when outside you tend to look farther than when inside? So distance vision gets used more and body adapts. Similar to how kids that spend time outside are less shortsighted.

I'm fairly certain I saw an article recently on HN that claimed it was not the distance focusing which helped but just the exposure to brighter light.

Edit: https://pubmed.ncbi.nlm.nih.gov/31722876/

Also, bright light will help the eye to focus. It's the same principle as if you were using a camera with a small aperture (and larger DOF, keeping more things in focus).