I wouldn't say that Mango's interest means that Kiwi was really disinterested. They are 2 different birds.
I have a conure and parrotlet as well. I'd say they are more different than they are alike.
But then, we are on our 2nd parrotlet, and he's very very different from the first. He's not as snuggly, not as adventurous, not as persistent.
Maybe focus less on what their differences may mean and instead go based on what you know or have learned for each bird.
Both Kingston and Nich get excited. But they show it differently. It doesn't necessarily mean that one is more excited than the other.