articles

I don't understand one thing.
I read some rules in my school grammar book and found one incongruity:
1) In certain cases nouns take the defenite article. It happens when the noun in apposition refers to a famous or well-known person:
Gorbachev, the political leader of the USSR, started the process...
2) If nouns in apposition denote a position (rank, post) which is, as a rule, unique and can be occupied by one person at a time, they are used without any article. Here belong such nouns as:
king-queen
colonel-captain
LEADER-head, etc.
E.g: Margaret Thatcher, Prime Minister of Britain, was the first woman to take this post.

But! There is one problem I can't understand: why Gobachev (THE political LEADER) was used with an article but Margeret Thatcher (Prime Minister of Britain) wasn't????????????!!!!!!!!!!!!!!!!!!!!!