Since this is not discussed in the award, any guess would be pure speculation, however, it seems very logical to me, here is why.
The PID is the baseline for pilot status pre-merger. The PID is the last measurement of position and status from the individual companies. Therefore it is the last known quantity not effected by the merger.
Now the fleet numbers in 2007 is a product of the merger and reflect how the company is responding to the merger, and can either support or disprove one sides proposal.
In other words the PID is a measurement of the pilots position and status. The fleet numbers in 2007 are what was the current measurement of the companies position and status into which you have to decide how to meter the pilots position and status.
Lets say by 2007 all the widebodies had gone away. Do you Nic would have still given the east the top 517? or just the opposite, what if the combined company grew by 50 airplanes and there were 500 newhires, do you think maybe then Brucia's concerns may have been given a little more weight, and some of the longer tenured furlough recalls would have been placed above Odell since Odell would have had some furlough protection?
The point being, why would you use the company's status on the PID, when you have the benefit of two years factual history. But, you have to use the PID for the pilots status, because any post-merger speculation of what would have happened stand alone is purely hypothetical.