React.memo — skips when props shallowly equal

What does this output?

function shallowEqual(a, b) {
  const keysA = Object.keys(a), keysB = Object.keys(b);
  if (keysA.length !== keysB.length) return false;
  return keysA.every(k => a[k] === b[k]);
}

let renderCount = 0;
function memo(Component) {
  let prevProps = null;
  return (props) => {
    if (prevProps && shallowEqual(prevProps, props)) return 'skipped';
    prevProps = props;
    renderCount++;
    return Component(props);
  };
}

const Child = ({ x, y }) => 'rendered:' + x + ',' + y;
const MemoChild = memo(Child);

console.log(MemoChild({ x: 1, y: 2 })); // first render
console.log(MemoChild({ x: 1, y: 2 })); // same props
console.log(MemoChild({ x: 1, y: 3 })); // y changed
console.log(renderCount);

Correct Output

rendered:1,2
skipped
rendered:1,3
2

Why this output?