I'm not a numbers guy, so I'm sure someone will come around to correct my terrible numbers, but the gist is this. The "soft cap" is the point past which defense "doesn't matter" unless you're facing enemies with defense debuffs. Because of the "streak breaker" code that ALWAYS lets 10% of hits get through no matter what, and I'm sure someone will come along with the actual formula, but basically each point of defense is worth 2% chance of "not getting hit", so 45 defense = 90% chance of not getting hit. So, anything beyond 45 defense is "pointless" unless, again, you're facing enemies with defense debuffs... which is why it's a "soft cap".