Excellent observation on your part, but I actually WASN'T having the broth appear out of thin air! (Although it certainly may have looked like that to Harry...)

 My logic went like this:

 Dumbledore did not transfigure the broth from nothing; instead, he CONJURED it directly from the Hogwarts kitchens-- the broth was already "pre-existing", as it were. I'd thought that the goblet itself would lend a clue, since everything that Dumbledore provided was conjured from Hogwarts-- bowl, broth, goblet of water, table and spoon. (Forgot to have him conjure a napkin for Harry!)

I do realize that "distance matters in magic", but this is Dumbledore we're talking about. The man conjured a chintz armchair during Harry's hearing before the Wizengamot (sp?) in OotP. Where did the chair come from? Presumably Hogwarts, all the way in the north of Scotland. London is in the south of England. Dumbledore has been described as the most powerful wizard of the age, so if he can manage to conjure an armchair all the way from Hogwarts to the Ministry, surely he could manage to conjure a bowl of chicken broth from Hogwarts to the Hut. (The location of the Rock is nebulous, but I imagine it to be a bit closer to Hogwarts than the Ministry.) That makes more sense to me than the idea that Dumbledore would have a ready supply of floral chintz armchairs stashed every so many miles around the Kingdom in order to be able to conjure the nearest chair in case of need! (I'm being serious, not facetious! Honestly! :-)  )

As for the broth, it has been established that food is available in the Hogwarts kitchens 24/7; and the elves can quickly prepare additional food to order, if necessary. I would extrapolate a bit and believe that certain foods might actually be kept in stasis to be INSTANTLY available, in case of emergency. Given that chicken broth (and, alternatively, beef broth) are commonly administered to people who have gone for an extended period of time without eating, before they start back on solid foods, it would make sense that hot chicken broth would be one of the foods kept in stasis, and therefore available for Dumbledore to conjure to the Hut.

 That was my premise for having him conjure the chicken broth, but it would have completely thrown the progression of the plot awry for Dumbledore to explain all that to Harry, whose eyes were already popping with shock!

Hope that helps clear up the matter for you, as to my way of thinking, but thank you for pointing out what COULD have been an error, if I'd not understood the principles of Gamp's Law.

Oh, shucky-darn-- I completely forgot to have Dumbledore vanish the conjured table!!! Egad!!

Thank you so much for reading and taking the time to review!!!  :-D