Notice how the digs are all from initial distribution blocks 9121 and 9122... Looking back further, I see that the digging took ~3.6% of the rewards from block 9128 (256 of 7120), then took 3.6% of the rewards from block 9127 (261 of 7120), then took 3.3% of the rewards from block 9122 (164 of 4984). Blocks 9123 through 9126 only had 356, 0, 712, and 356 rewards each, so I didn't bother checking the percentage claimed from those.
So it looks like this guy, whoever he is, is able to claim around 3.5% of the rewards in each block, and is working his way backwards through the blocks. According to xploited, the initial distribution ordered the rewards randomly, so there's no reason to think that he won't be able to claim a similar percentage of the rewards from all the blocks.
Regarding the discussion on why he might be digging the clams in this fashion, one of the Dev's mentioned that it would be more difficult for them to do it this way because of the randomization, and the assumption is that they're doing it this way to perhaps send a message to us of their motivation. I disagree that it would be more difficult.
I wonder if they didn't just grab a dump of the whole CLAM blockchain up to the last block where the initial distribution took place, exported all the diggable addresses in the order they were funded, and is just backtracking their way through the list while matching each funded addresses to ones that they own. This makes the most sense to me, anyways.