Problem with categories

Hi all.

My site is http://www.skakistiko.com/

The problem is that when I click on a category (next to the post header) or in the dropdown menu (widget), it lists all the entries, from all categories.

I click on a category with one entry, it lists all the entries etc (e.g. http://www.skakistiko.com/?cat=104 – with only one post)

Any idea how to fix that?

Thanks in advance.