I am seeing a lot of code that explains how to centre a subview inside a view. The code examples typically go like this:
SubView.center = view.center;
So when setting it, it is setting it inside it's superview. When getting the subviews center it gives you the actual views center.
So half of 50 is 25, hence 25,25. You are wanting the subview's center not its parent's center so there is no reason for it to return its parent's center coordinates, just its own.
To be a bit more technical it has to related to Frame and Bounds of a view (the getter is getting the center by using the view's bounds and the setter is using the view's frame). Here is a link that describes what those are and how they are different.