I had been thinking that perhaps my explanation of how the two axes become one axis in people’s minds was lacking. So, here is another go:
Imagine you’re starting a completely new endeavor. Before getting to work, you consider the risks and lay them out in a matrix. This matrix will generally fill a square though the distribution of risks through the square will likely be uneven. That’s ok, just picture the square as the overall field. Now, before starting, you’re going to work at mitigating the risks. You’ll likely focus on the upper right corner as these are both highly likely and highly consequential. As a result of your efforts, while you’ll still have residual risks from these, you’ll be displacing them down and/or left. This means that corner is getting chipped away before you even start your new endeavor. After you’ve started the endeavor, you’re going to experience some of these risks. As you encounter the hazards, you gain experience recognizing them earlier and at adapting to counter and/or avoid them. You tend to push down on the likely of the entire square. You also push the most damaging leftward as you gain experience, but this is uneven as you encounter the more likelies of the more consequential more often. Thus, you keep knocking the right corner away. Over time with your risk distribution you’ll find instead of having a square, you have a ramp with a downward slope. Thus, you grow accustomed to that line as your axis of concern rather than the two axes. It is experiential and you’ll need to remind yourself the axes are “independent,” and should be viewed orthogonally. Don’t focus on the ramp. Don’t let that become your axis of reference.