Yes, its part of the oddities of tennis history, and i confess that i have no definitive solution. I have always believed that Laver won 5 US pros, and Rosewall 3 (including his 1971 win at Boston), Gonzalez 8 and even Borg 3. And so stand the record books. Also Wembley was continued after 1967, and Rosewall won one more in 1968 (called Kramer Tournament of Champions) and Laver another 2 in 69 and 70, when it was called British Covered Courts. Its especially difficult to decide what was a biggie in the early open years, when many open majors (with the exception of 1969) had weak field without most top contenders due to the political struggles of promoters and federations.
Even if we don't consider the events after 1967 for the MAJORS balances, it's clear that Laver won 5 US Pros and Rosewall 3 altogether. But it's a significant difference between majors till 1967 and "normal" events in open era.
For instance Rosewall's 1971 US Pro win is important for his 1971 ranking (a tough tournament win) but not for his all-time majors sum.
Note: I do not think so because it's "better" for Muscles. I would think so also in the reverse case!