An otherwise non-Classics-related review in the Washington Post begins:

In the middle of one of the most gripping action sequences in the Aeneid , Virgil deliberately calls attention to the artificiality of the story he is telling. It occurs in Book II, in the account of the sack of Troy. Virgil first says the Trojan horse is made of fir; a hundred lines later, he says it's made of maple; next it turns to oak; and, still later, it's pine. Not only does the horse's protean essence function as a metaphor for the inherent deceptiveness of Greek gifts, it serves to remind us that we are hearing a tale told to the Carthaginians by one very interested participant -- Aeneas -- thus alerting us to the presence of more subtle fabrications
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I'd never heard that explanation before for the 'varying wood thing', but it sounds like the sort of thing at least one Classicist would come up with. What always stuck in my mind from my undergrad dalliances in Virgil is Williams' comment (ad. 2.16), "... this may be taken as a sign of lack of revision or lack of special interest in carpentry."