What about a third possibility: that they already had parity beforehand about many issues important to them? Most wisdom about successful marriages I've heard is that you should marry someone whose values are aligned with your own.
This depends on how you define "personality" and "values". Because for example, according to psychological research, personality traits (as defined by the Big 5 model of personality) are strongly correlated with things such as political beliefs (which could be argued is a subset of a person's values at least), and are largely stable over time. So it wouldn't be too much of a stretch to suggest that the more similar the personalities between a couple, the higher the probability of them also having a large set of overlapping "values", however you choose to define them.
Not every point of disagreement is important: not every value has relevance to a family and not every value one holds is equally difficult to compromise. It's more important to agree on things like finances, children, moral/ethical standpoints, etc... than on politics, music, or hobbies and interests.