NodeJS: calling global.gc() doesn't reduce mem

Question

To investigate memory leaks, I have setup a route that triggers global.gc() at every POST /gc

app.post('/gc', function(req, res){
     global.gc();
});

However, I've noticed that if I spam this request, it reduces the memory usage more and more each time. Shouldn't calling global.gc() once is enough to reduce the memory to minimum?

If so, why does calling multiple times consecutively reduces memory at every call?

(I'm using Node.js v0.12)