Did I do something wrong with the supports in Fire Emblem Three Houses?

Question

I was playing Fire Emblem Three Houses and I beated the Blue Lions route and I was anticipating what happens with Dimitri and Marianne (I got them to A rank and I loved their supports). When I got to the credits, I was surprised to see that I got Dimitri's ending with Dedue instead of his one with Marianne. I checked and saw that Dimitri also had an A support with Dedue. My current theory is that some supports take precedence over but I'm not sure.