A good partnership is not just about a size, skill, and style match: it's about work ethic, attitude, perseverance, collegiality, and so much else. Those factors are impossible to quantify and compare from competition performances or news articles, but the pair community is small and everyone knows pretty much everything about everyone else.
The singles elements in a pair program do not exist in isolation from pair element. The real challenge - what's really hard about stringing together elements into a program - is the necessity of dulling the cognitive processes and allowing muscle memory to take over. XXX doesn't have the muscle memory of the pair elements to be able to do them without thinking about what she needs to do, especially because they have changed the technique on the elements that they did last year, so her muscle memory was restarted. She does have the muscle memory to be able to do 3T/3S in her sleep, but I suspect that was her undoing: in a program where 11 of the elements are new to you, it's easy to either underestimate or overestimate what you need to put into the two jumps. I could still see her inexperience in pairs - the tracking is considerably better than at Nationals but still not all there - but I also saw considerable improvement in their elements. It's hard to both rework all elements and learning perfect tracking in one off-season.