Why aren't you testing these with a 0/0/0 spec?

A couple of reasons: time constraints, and because 0/0/0 would neglect Disc Mastery. Either way, I've verified the numbers with Disc. I will test 0/0/0 when I get a chance, but testing with 0/0/0 would only narrow down the reason why it's different.

Also Enlightenment is the reason PW:S's absorption goes up simply by switching to Discipline.

Enlightenment is already baked into SP values on the character sheet, so it's not the cause here. I'm already using these values in my calculations. Disc also has its Mastery bonus, of course, which affects the absorb value.

I've tried to model the discrepancy in a number of ways:

Y = ( 3906 + A * SP ) * 1.1. * 1.06 * MA -- Assume base is correct, coeff is wrong

Y = ( A + 0.418 * SP ) * 1.1 * 1.06 * MA -- Assume coeff is correct, base is wrong

Y = ( A + B * SP ) * 1.1. * 1.06 * MA <== This model seems to be the correct one

Y = ( A + B * SP ) * 1.1 * 1.06 * MA + C

Y = ( A + B * SP ) * C * MA + D

The first model already tries to account for just the SP or coefficient being affected differently. If there were some difference besides just the +15% int bonus from Enlightenment, the first model would have caught it.

The last one would have been the most accurate, but the non-linear regression tool I used (http://statpages.org/nonlin.html) couldn't get a convergence, so I gave up on it and simplified it from there. The third one was the one I used to arrive at these results.

As I said, it's also possible that the Imp PW:S talent is causing things to change, rather than being specced Disc. That's why in another thread I asked if any Holy priests specced into Imp PW:S could post some numbers, because ATM I'm in no position to do so.