Well, the results are exactly because you assumed a defense value of two, to armor two and block four, which makes the overall damage reduction on a successful hit equal. You'd get a rather different chart if, say, that was defense one, or defense three, or etc.
Let's consider another example. You're under attack by a super-powered ninja (fists 5), who's got inhuman strength (making those fists attacks count as weapon: 2). You're a moderately competent evoker, capable of casually calling up a block six or armor 3 shield, and you've got a defensive item worth block 8 or armor 4 if you choose to activate it... but your athletics is merely a 3.
Average attack - you roll a zero for defense, and the other guy rolls a zero to attack.
He hits with two shifts, plus two for weapon rating, for a total of a four stress hit. On the other hand, if you have up any of your block shields, he's going to miss. If you've got armor: 4, he hits, but does nothing; if you've got armor: 3, he hits for one stress.
Worst case attack: you roll a -4 for defense, and the other guy rolls +4 to attack.
He hits with ten shifts, plus two for weapon rating, for a 12 stress hit. If you have block six up, that's a mere five stress hit. If you've got block eight up, it's a three stress hit. Armor, on the other hand, would only reduce that by three or four points, leaving you with at best an eight stress hit to deal with. Or, if you've already got a block six shield active, you can activate that defensive item for armor and take only a one stress hit; if you've got an armor three shield already active, you can also use the defensive item for block 8 and take nothing. Though, in all of these cases, whatever block you're using is going down after that one hit.
On a best case attack, of course, he just plain misses, and it doesn't matter what defensive spells you've got running.
There are, of course, a lot of possible intermediate grounds inbetween these three scenarios.