Not massaging figures but Alliance are unionists and a majority of their vote is captured in areas where nationalists do not make up a significant number, e.g. East Belfast. Whether they want to categorise themselves as unionists or not it is purely to create from clear water between themselves and the UUP where they fight for the same vote. They are occupying seats in the Assembly that would previously have gone to the UUP.
BTW I am not making a point with these figures other than to explain why the unionists have more seats than nationalists, i.e. they can turn out more voters than nationalists. Even adding the Alliance to the other group makes little real difference to a unionist majority of the votes being cast:
Nationalist 241,407 38.56%
Unionist 329,688 52.66%
Others 55,030 8.79%
Unionists still have more than half the votes cast.