A couple things. Remember that his total DKP is Total Earned - Total Spent. That means that even though he's got raids that are fully decayed, those old decayed raids are still worth 25% of their original value.

Now, you say that he hit zero at 11/2. Remember though, hitting zero doesn't mean that decay stops. You CAN decay into the negatives, because ultimately everything is decaying.

So, the difference you stated between the 104 and the 71.46. This ultimately means (assuming you are correct that he hit zero on 11/2) that since 11/2, he has decayed another 42.54 points (42.54 + 71.46 = 104).

If he had STOPPED raiding on 11/2, he would have continued decaying into the negatives.

Now, the REASON you continue to decay into the negatives is this:

EVERYTHING must terminate at the minimum value. If you hit zero, but you still have raids that are not fully decayed, the decay continues until those are fully decayed.

Here is an unintended problem with the DKP decay, and this is what triggered the controversy.

Person A has been raiding with the guild since day 1. Rarely misses a raid and has made some purchases along the way.

Person B comes in the guild 6 months after DKP has started. Rarely misses a raid, has made 1 purchase long the way.

Because person A has been in from the beginning, he is actually earning less DKP week to week since he is affected by decay on DKP earned prior to person B entering the guild. This in turn causes person A to want to horde their points since Person B passes them up in current DKP balance. This is the complete opposite of what, I believe, was intended since this actually punishes people that raid consistantly for extended periods of time.