This survey implicitly assumes that most (if not all) public speeches of all these PMs were made/are available in public domain which certainly isn't the case. As far as election campaigns go, I don't think "most" of those speeches could now be seen/listened anywhere, at least before Atal Bihari Vajpayee.
Technology and media coverage/bias must be taken into account for a survey like this.
The speech writers must be writing pretty well but because he just doesn't have sense to talk in public as he doesn't understand the ground reality. And this isn't the first time he has said something regretting or baseless; that has always been the case since he started giving speeches.
Just because he inherits the Gandhis doesn't mean he's fit for the PM position. Don't know when people will understand this.