For the "achievements" that Hun Sen accoplished, former King Sihanouk gave him the title. But it doesn't mean that Hun Sen is equivalent to King Sihamoni today. King Sihamoni's title is Preah Karona Preah Bat Samdech Preah Boromneath Norodom Sihamoni. In shorten form, one could just say Preah Bat Sihamoni. Hun Sen could never be reffered to as 'Preah Bat.' He is just Samdech Hun Sen.
Khmer royal titles are very complex. Samdech is given to very high officials in Cambodia. Samdech is also given to great monks in Cambodia (e.g. Samdech Chuon Nath is highly respected for the things that he did and there is even a holiday for him). Today, it could be compared to the term 'knight' in England, however it is not given out as much.
It should not be confused with 'Sdech' which means the king. Non-royals cannot be called 'Sdech.'
Here are some royal titles:
http://www.4dw.net/royalark/Cambodia/glossary.htmBy the way, Khmer also derive some terms from Thai too, such as Fa (or Fea) and Chau. Like in Chau Fa.