Why is a joke?

In Obama's Westminster Hall speech, he began with:

"My Lord Chancellor, Mr. Speaker, Mr. Prime Minister, my lords, and members of the House of Commons:

I have known few greater honors than the opportunity to address the Mother of Parliaments at Westminster Hall. I am told that the last three speakers here have been the Pope, Her Majesty the Queen, and Nelson Mandela -- which is either a very high bar or the beginning of a very funny joke."

Everyone laughed here. I don't know what is funny here. Could anyone help to translate it

into Chinese or explain ?

Thanks.