1 isn't a matrix. Integers/floating point numbers are in many languages treated differently than other types for reasons of performance and memory management etc. Ruby isn't different in this respect (IIRC they call it immediate objects). A matrix is a "normal" object in ruby.

BTW the ruby equivalent for the above convention would be (maybe inspired by scheme?)

    a.inverse!() -> mutate a
    a.inverse() -> return the inverse

Which one is more consistent?

This confuses me, because python strings use str.lower() to return a lowercase copy of str, leaving str unchanged. I prefer the Ruby idiom of naming the mutating versions with a !.