The fundamental rule is that all play counts towards world rankings not just the world tour. The pro majors and tournaments are just as important as the world tour. A pro major, like a modern slam, requires a player to win on the big, important occasion when it really matters against all the best players. Its an important skill different from the consistency skill required to win a tour. The pro majors allow all pros to compete for the world number one spot not just the restricted number on the world tour (somtimes as low as 2). L'equipe Ranked rosewall number one in 1960 and 1961 so winning the world tour was not only thing that mattered. The sunday times considered Rosewall the dominant pro from 1959-64. I do not agree with this but it again shows that winning the world tour was not sufficient on its own to get the number one spot.
The 1964 tour was a different tour than the earlier head to head tours because it was a series of 8 man tournaments linked together. Its more like the Grand prix of the 1970s. Winning the Grand prix did not make Richey (1970), Vilas (1974-5) or Ramirez(1976) a contender for the number one spot. The majors and other play like the WCT tour counted as well. The 1964 tour win for Rosewall is not that significant either. That tour did not give extra weight to the pro majors nor did it cover all play (it only covered 19 events not the full 31 events actuall played). The fundamental rule is that all play count for world rankings. On overall play Laver won more tournaments (11 to 10) ,won more majors (2to 1), crushed Rosewall 15-4 in the head to head and Had a superior win percentage (74.8% to 69.5%). On all the standard world ranking measures was ahead so I have no doubt that he should be regarded as the undisputed number one of 1964.