Why is the method being called from the interface instead of the implementation?

There are several things to understand here.

First and foremost, you simply can't call an interface method if it isn't implemented somewhere. Or to put it another way, a class that implements a certain interface must implement all methods from the said interface or the class won't compile. So, be sure that we are actually calling the implemented method in this case.

Now on to the details for this particular case. The CategoryDao object we have in the CategoryServiceImpl is Autowired to get an instance of a class implementing the CategoryDao interface, it can't instantiate an object of an interface. And it's the way Autowired works, put an interface as the type and then Spring looks for a class which implements that interface to instantiate the actual object from. The class which implements the particular interface also needs to be annotated with Repository (or any other Spring component). So, since there's only one such class, which is CategoryDaoImpl, the object we have is an instance of that particular class.

So what happens behind the scenes looks something like this:

CategoryDao categoryDao = new CategoryDaoImpl();

And when we call methods on that categoryDao object, we are actually calling the method implementations from CategoryDaoImpl.