[susan53]

Yes - good one. It happens with all other titles too :
She is Head of the Science Departmant at the local school.
He is Professor of Economics at XXXX University.
She's Assistant Marketing Director at XXXX.
She was crowned Queen of the Netherlands in 19XX

Again, the use of the article is optional - it can be included but is often omitted.

So, two diverse categories here : nominalisations and titles. Back to the drawing board....