Say I had a regular door that I could either open or close.

What are the odds of either? 50% chance I'll open it and 50% chance I'll close it. Now, what if my primary reason for opening the door is to walk through it? That would change the odds, no? I would be opening the door to go through it. But we all know that I wouldn't fit through if I only opened the door by 10cm.

What are the odds of me opening the door wide enough to ensure I will fit though it? The 50/50 chance of me opening/closing it is still present, but with the added technicality of me completing my goal (walking through the door).