I think you have explained it yourself - the tree is a bit sharper than the cat.
Therefore you focused on the tree, you may have meant to focus on the cat, you may have done your best to focus on the cat, but you actually focused on the tree.
It is what experienced photographers know as a bit of a mistake ( or an artistic statement if someone asks
)
It happens to all of us - more often than we would like to admit - its pretty easy to do, especially when you are sure that you have the focus right and stop to think of something else.
Neither tree or cat is central, and I don't know how you focused, but if you used auto focus it probably gave most 'weight' to the centre focus spot and that seems to be lined up over some plants at the same depth as the tree - then with a wide aperture to try to blur the background you have slightly blurred both back and foreground including the cat.