I think it's multiple things. Your eye is a muscle, focusing only at close range keeps the range of motion tight. Just like your back and neck might get stiff

I think it's multiple things. Your eye is a muscle, focusing only at close range keeps the range of motion tight. Just like your back and neck might get stiff sitting all day. Sunlight means smaller pupil, wider depth of field, just like a camera. (My speculation) More light gives you brain more information to clean up or upscale a blurry image. This effect even seems to last when you leave sunlight, for awhile. It's like I fine tuned my neurons to prioritize blur reduction, and it takes awhile for them to fall back into normal processing. If this is true, my guess is that I might be trading off contrast or color details for sharpness.