Your viewfinder is probably not accurate.
You'll probably get more of a scene in your photograph than what you saw through the viewfinder.
The only cameras with 100% accurate viewfinders are the top-of-the-line models.
To see the inaccuracy, take a picture of a rectangular object, like a painting hanging on the wall.
Carefully line the edges of the painting up with the top and sides of the viewfinder.
Remember where the bottom of the frame crosses the scene.
When you look at the photograph you took of the painting, you'll see where you're getting more than what you see through your viewfinder.
You may get more on the left, the right, or the top, etc.
When composition is critical, you'll be able to adjust for viewfinder inaccuracy.