Arun,
Another good question.
I don't think you are necessarily missing anything. Your calculation is correct.
The Cut Off Dates chart uses a combination of known backlog from the DOS Demand figures (prorated to the monthly splits in the USCIS Inventory, which is slightly higher) and adds PWMB based on PERM using the formula above.
PWMB are defined as those whose PERM was Certified after August 17, 2007 and therefore could not have filed under the July 2007 VB. August 2007 is therefore a month where the figures are a combination of those known and those calculated.
I would have been amazed if the figures had matched exactly. It is within about 2%, which I would deem more than acceptable.
Whilst we all try our best, do realize that it is a fairly inexact science, because the data isn't great or necessarily complete.
It probably is worth reiterating to people that potential error margins are quite high, due to having to make multiple assumptions. That is why I didn't want to introduce another one for I-485 denial rate.
The numbers should be treated as qualitative and indicative, rather than quantitative.
I really don't mind if people don't agree with the assumptions, because their own might equally be correct.
On a limited basis, if someone has a strong opinion about some different numbers for the assumptions (with some explanation as to why), I am happy to run those figures and report the result to them.
Truthfully, no-one knows the real number. If they claim otherwise, they are either a liar or deluded

. I certainly don't.
All I try to do is put my best thoughts out there, with some explanation of how the figures were arrived at.
PS - That reminds me, I need to upload some very small changes to the figures. For the technically minded, it is the difference between half of April and the 16 days that are actually left. It is only 28, but the numbers posted on the forum are different from those in my spreadsheet.