The venom is a toxin that actively attacks cell structures, causing necrosis. Some spiders bite once, but others bite repeatedly while they are able - macerating the skin and allowing venom to more easily spread through the subcutaneous tissue. I'm not sure if the brown recluse does this.
This page describes how bites from that spider can take 3-4 hours before they are noticeable, and discusses reasons why spider bites in general are difficult to diagnose and therefore may get a bit... icky.
That snopes page I posted earlier did not come to any clear conclusion but it does say:
I can see how someone might get bitten, dismiss the bite, then when it gets itchy and sore a few hours later, it gets infected by their scratching.
Whoever it was in that photo was certainly unlucky