[basic.fundamental] p3 doesn't define "corresponding bit" either, it also just uses it. And that seems fine to me, this is normal English usage of the word. Since there is nothing to suggest otherwise, it seems obvious to me that the first bit in the value representation corresponds to the first bit in the object representation, and the second bit in the value rep corresponds to the second bit in the object rep etc.

For example, Merriam-Webster gives this as a definition of corresponding: "having or participating in the same relationship (such as kind, degree, position, correspondence, or function) especially with regard to the same or like wholes (such as geometric figures or sets)" (emphasis mine). We are talking about an ordered set of bits here, and the correspondence is the position in that ordered set.

Assuming that some other correspondence (reverse order, or arbitrary order) when there is nothing to suggest it seems like a particularly hostile interpretation.

I agree with @jwakely here; "corresponding" seems clear enough. (There are other uses of "corresponding" in the standard that are also based on the plain English meaning of the word.)