I'll take a shot at this, though I'm afraid it may lack elegance.
Permission and Deficiency are both related to how much you have of something. If you have too much, you run up against Permission, which establishes limitations. If you don't have enough, you run into Deficiency, telling you what's lacking.
Need and Expediency are more worried about the actual stuff. Under the dynamically opposed Concern, in the same relative positions as Need and Expediency, you see Situation and Circumstance. Situation and Circumstance both look at the same thing, each through a different lens. Situation is stuffier, more reasonable and objective, and Circumstance is more emotional. Need and Expediency have a similar relationship. Need says, "I have to do this because it is required." Expediency says, "I have to do this... or else."
The hailstorm of the century is coming. You have two cars but only a one car garage. In other words, only one car is Permitted. The garage has a mass of one. Deficiency steps in, complaining that's not enough--something will have to change.
Need assesses the situation and gives you your options. You need to expand the garage or get rid of a car. Increase the Permission or eliminate the Deficiency.
Meanwhile, Expediency lies on the couch with the back of his hand thrown across his eyes, reminding you that OMG you'll be eating ramen and tuna for half a decade if you pay for a new garage, but your new girlfriend really likes that convertible, but you need the truck for work... maybe you should just drive one of the cars under an overpass and wait out the storm...