Atmospheric conditions change. Looking out my window right now there is a very slight drizzle, combined with a little woodsmoke buildup from lack of any breeze. I see more "haze" looking at some trees 300meters away at the moment than I do with clear conditions looking at a hill top 8 miles away (which isn't visible at all at the moment).
Hills and mountains also appear smaller the further away I get. The sun appears the same size all day, despite being 2-3times (or more) further away at sunset or sunrise than at noon.