Sometimes the two terms have the same meaning and can be interchanged, but other times there is a difference in meaning. That is, 'Nationality' sometimes is used to refer to a person's place of birth or origins, whereas 'citizenship' doesn't imply this. Citizenship solely says you have certain legal rights granted to you. So if I said I was a french national, but an Australian citizen, I am implying I was born in France or my origins are there but I have the 'rights' of Australian citizenship. BUT, often the two terms are interchanged, so context of the document is important.

Thank you, Manek!

I think it is to give more clarity. They mean the same for all practical purposes.