In SLR photography, why are some large aperture backgrounds blurred, while others are not so blurred, just a little blurred?

In general, the use of large aperture is mostly used for portrait photography, focusing on the face or eyes. In the case of focusing on reality, the front and back background will be blurred (some people say it is blurred), but the plane corresponding to the face or glasses will be clearer. If it is a close-up in the landscape, the focused object is clear and blurred before and after. Unless it is out of focus, or the object of framing is almost close to a plane, it will be basically clear.