If you search the forum here, you will find that if prof. that interviews you is not from your specific area of interest, than probably you are an "almost" candidate and the admissions committee is not sure whether to admit you or not.
If a prof. from your research area interviews you, than you are on paper a good match and are considered for funding via fellowship.
The above are general assumptions, and it could also just be that the prof that will interview you is accepting students and wants to talk to you if you would be interested joining his research, even that you specified other interests.