It's because it is also linked to temperature, the higher your pressure the higher your boiling point. It also effects how fast the fluid travels through the ground coffee which effects not just how strong it is but also can have an effect on the balance of what has diffused from the ground and various other characteristics.

There's a lot that can go into making a good cup of coffee. New Zealand (where I'm from) has one of the higher coffee shops per capita in the world (mostly non chain) and you may be surprised how much a coffee can change from barista to barista even using the same beans in the same shop. If you're spending the money every day on that little moment of luxury you can notice the difference. (Whether or not it's suffering from the same biases in experience as expensive wine can I'll leave for someone else to speak on).