Daisy - nice!
You've done well to photo the swallowtails, especially atop those tall verbena, probably swaying in the breeze.
I've watched swallowtails feeding in my garden and it seems to me they tend to keep fluttering while feeding; I've wondered why; perhaps their weight might bend the flowers down. When they are ovipositing, they are much calmer. But anyway, nectar is obviously a very rich source of energy - some butterflies (and moths) do a lot of flying versus the amount of time they spend nectaring. So one might argue that their high-octane fuel allows them the luxury of being fussy - perhaps ignoring some flowers nearby for others further away. And if those further flowers are the best nectar source locally, then so much the better.
I think there are lots of aspects of lepidopteran feeding behaviour that we don't understand. How does a butterfly remember which florets of which flowers it has already drunk the nectar from? (if it does). Why do butterflies favour certain flowers over others? (not just a question of ultraviolet honey guides). How do they know when to give up on the flowers in one locality when another locality is becoming more nectariferous (if there is such a word)? Many such questions assume a certain efficiency in their behaviour, which may or may not be valid.
I do not know the real answer to your question, Daisy.
But I do know that first we have to be curious, then observant, and then formulate a question. You've done all of those.