runtime get vs. immediate get

gistfile1.txt

function Class()
{
    this._a = 0;
    this._max = 0;
}

Object.defineProperty(Class.prototype, 'a', {
    get: function() { return this._a; },
    set: function(v) {
        if (v > this._max) {
            v = this._max;
        }
        this._a = v;
        return v;
    }
});

var c = new Class();
c._max = 1000;

requestAnimationFrame(run);
function run(ts) {    
    console.log(c.a+=10,ts);
    requestAnimationFrame(run);
}