It’s hard to find spots to record nature soundscapes that don’t have some kind of traffic — planes, highways, motorboats. If it’s distant enough, I might barely notice the distant drones. A recording makes it much more obvious.
I haven’t scientifically tested this but I think both microphone focus and psychology create this effect. Psychologically, we have a natural ability to focus on what we’re trying to hear and we filter out the extraneous sounds, as long as they don’t drown out what we’re trying to hear. Later, when we listen to the recording, the recording itself is the object of our listening and we hear all the sounds at a more equal level. At the technical level, depending on the type of microphone, There’s also a telescoping effect that brings out the distant, less desired sounds.