Hi
There is nothing to fix here actually, it's by design. "MOUSE DOWN" global event is the direct Unity call, while "Mouse Pick 2d Event" action wraps mouse inputs and adds some features to make it more convenient.
what do you find confusing? It's important that these concept are totally digested, so don't hesitate, I am willing to explain or word differently, I can do a scene to show how it works as well if that could help.
bye,
Jean