Conjuring food

Supposedly, food cannot be created by magic, only duplicated or multiplied if you actually have food already. This jives with HBP, when Harry was able to refill the wine bottles for Hagrid and Slughorn because he noticed the wine was running out and acted before it was entirely gone. But I don’t get why our hero-trio was starving while camping in DH? They could have accio’d a single egg (or any small food item) from a Muggle market and expanded it, so that it fed all 3 of them. Make it make sense, please!