Because as the quote said, it depends on the client requirement.
"if you click anywhere in the left half of the screen during the 5th screen all the settings get deleted" might be a very severe/critical bug, but if the client says "thats fine for me don't worry, but can you fix the wrong color on screen one where the logo is purple instead of fuschia that's not acceptable for my needs" then this one has a higher priority, even though it's much less severe.
Your job is not to build the perfect/best product, it's to build the one your client want and is willing to pay money for.
The mixup between priority and severity only comes from the modern era when so many product get made for internal consumption, or where the user is not not the buyer.
"if you click anywhere in the left half of the screen during the 5th screen all the settings get deleted" might be a very severe/critical bug, but if the client says "thats fine for me don't worry, but can you fix the wrong color on screen one where the logo is purple instead of fuschia that's not acceptable for my needs" then this one has a higher priority, even though it's much less severe.
Your job is not to build the perfect/best product, it's to build the one your client want and is willing to pay money for.
The mixup between priority and severity only comes from the modern era when so many product get made for internal consumption, or where the user is not not the buyer.