There’s not just a few parallel texts: there’s thousands and thousands of passages, phrases, paragraphs, at every level of the texts. The incredible thing is to think about how it was possible to coordinate this material over such a huge scale.
Clearly there was coordination among the nikaya compilers. But there are also differences. In scope and aim, as you mention, but also in details of phrasing, how abbreviations are handled, and similar matters.
An outstanding example of this is the curious way that the six senses are abbreviated in the Salayatana Samyutta. In the first few suttas, we have the six senses presented, and it is abbreviated as expected. But from SN 35.7, we see that the jīvhā is treated in full, while other items are not. The same thing then happens repeatedly throughout this collection; but not, so far as I know, anywhere else. Why? I have no idea!
When thinking about these things, we should bear in mind two opposing dynamics, both working all the time. There is a tendency to dispersal, to diversity, with individual monastics, monasteries, and later, schools, all diverging in their own way. Then there is the centering tendency, most obvious in the traditions of the Councils. The traditions want us to believe that the centering tendency is the most important one; but it is more complex than that. The strength of Buddhism is in the tension between these two trends, each balancing the other.