I think Frank has said it all. Whether an "ordinary" English speaker would consider a statutory or constitutional provision ambiguous is irrelevant to whether a judge or other lawyer would consider it ambiguous. The more one knows about a subject, the more--or less--ambiguous a document relating to it might appear. An ordinary English reader might think "freedom of speech" in the First Amendment unambiguous; a lawyer knows better.
Phrases and especially sentences are meaningful only in context, and different types of reader have access to different elements of context. Contract lawyers distinguish helpfully between intrinsic and extrinsic ambiguity, where the former can be spotted with the aid of a very limited knowledge of context, while the latter is discovered only with a broader inquiry. So in the Raffles case, the contract to deliver cotton on the ship Peerless was unambiguous until one learned that there were two ships of the same name sailing between the same ports at roughly the same time (though the difference in date of arrival was what made the ambiguity costly). That is the classic example of extrinsic ambiguity.
In Chapman the major ambiguity stemmed from the weight of the cardboard relative to that of the LSD and the arbitrary consequence (or so it seemed to me) that if the cardboard was treated as part of the mixture or substance containing the LSD the sentence would be much heavier.
When judges say that the "literal" meaning of a statute should be followed unless the result is "absurd," they mean (or should I think be understood to mean) that the statute isn't clear once the context is understood.
And another way to put all this is that a statute is a communication and that in decoding a communication we draw on everything we know about the communicator and the subject matter of the communication in deciding what it means.