It is the Release call that calls the destructor and frees the memory when the refcounter reaches 0.

The garbage collector is only there to help resolve circular references, i.e. when the refcounter would never reach 0 under normal circumstances. The side effect of this is that the garbage collector has to hold a reference on its own for objects that may form circular references. When the garbage collector sees that the refcount is 1, it can immediately release its reference and thus destroy the object without further checks.