As I said before, Tidus' pyreflies could be different from Auron's. Also it says the Guado are keen to the scent of the Farplane, not of those from Dream Zanarkand. Everyone and almost everything is made of pyreflies, Fiends, people, even the Aeons themselves are Pyreflies.
However, if you'll notice, Yuna never performs a Sending for Fiends.
This is proof that pyrefly constitution varies from object to object.
It is possible that the pyreflies that constituean AeonTidus are somewhat different from the pyreflies of the unsent [Auron], that perhaps when they form a material existence, their bonds are stronger (think of it as the difference between a covalent's and an ionic's atomic bond...)