What's the keyword mutable in the Moses source code for?

This is about the source code of the statistical machine translation system Moses. In the Factor class of the Mosesdecoder project, there is this strange use of the keyword mutable:

class Factor {

    __SOME_OTHER_CODE__

    // FactorCollection writes here.
    // This is mutable so the pointer can be changed to pool-backed memory.
    mutable StringPiece m_string
    size_t m_id;

    __SOME_OTHER_CODE__
}

The complete file of Factor.h is here. I know that mutable is used when you want to modify a member variant in a const member function, or when you want to modify some out-of-scope variable in a lambda expression. I do not, however, understand what the mutable is doing in this code.

I appreciate any hint. Thank you.