I rarely notice trivial changes these days since they are little more than change for the sake of change. Whether the change was made by bored UI designers, to keep up with the latest fashions, or to give a visual indication that the product is being updated is something that only the developers can answer.

That being said, it can be more difficult to differentiate some inactive tabs. Some tabs lack a favicon (e.g. PDF files) and the close button blends into the text.