Historically, the name for a weighing device is scales. It is
plural because scales had two clear parts in which one thing was
weighed against another. So, they looked like this: Modern
scales don’t involve balancing things in two plates (photo from
here): (b)AmE has changed along with the scales, so that item
(b) is usually called a bathroom scale, but scales is still used
for the older kind. In BrE and AusE, however, it is still called
scales, no matter whether it has two salient parts or not. When
Anna Wierzbicka (Semantics: primes and universals) asked
Australians why the word is plural, they answered that it was
because there are lots of little numbers on the contraption.
This seems to be a case of the word leading the thinking about
an object. That is, because they say scales instead of scale,
some people think about scales as being 'made up' of little
numbers because they need to make sense of the fact that this
singular object gets a plural name. Wierzbicka also notes that
Australian English has shift