The ANSI definitions are bad: they allow multiple interpretations with varying results. 25 years ago, Berenson, O'Neil, et al. published a paper showing the ANSI definitions had this ambiguity, and that what the spec meant to define should have been a broader class of anomalies. They literally say that the broad interpretation is the "correct" one, and the research community basically went "oh, yeah, you're right". Adya followed up with generalized isolation level definitions, and pretty much every paper I've read has gone with these versions since. That didn't make its way back into the SQL spec though: it's still ambiguous, which means you can interpret RR as allowing G2-item.
Why prevent G2-item in RR? Because then the difference between repeatable read and serializable is specifically phantoms, rather than phantoms plus... some other hard-to-describe anomalies. If you use the broad/generalized interpretation, you can trust that a program which only accesses data by primary key, running under repeatable read, is actually serializable. That's a powerful, intuitive constraint. If you use the strict interpretation, RR allows Other Weird Behaviors, and it's harder to prove an execution is correct.
For a very thorough discussion of this, see either Berenson or Adya's papers, linked throughout the report.