First of all, as per the backlog reduction plan, the target for FY2004 (by Sept'30) is 20 months average cycle time , not 20 months backlog time.
The average cycle time is defined as "Avearage time taken for processing an application over the last 12 month period". The fact that they have taken more than 24 months for most of the cases during the last 9 months is going to hurt their average. The current average, at the end of 9 months of FY2004 stands at 24 (as for official records it is 23 months as in April).
Now to bring this average to 20 months, they can do 2 things. Make a rapid improvement , may be 10 fold increase in the pace of approvals or approve a large number of newly filed cases so that they will bring down the average. They are following the second choice

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Here is the sample math that explains their strategy.
During first 9 months, they have approved 100 cases with average processing time of 24 months. Now let's say that they have not improved the pace of approvals at all. So if they approved 100 cases in 9 months, they will approve 33 cases in next 3 months (because they didn't improve their speed at all). Now comes a chance for them to still bring down the average to 20 months without making slightest extra effort. How ? Well, simply by approving newer cases in the next 3 months. By approving a lot of newly filed cases, they can make the average processing time for these 33 applications as 6 months.
Final average at the end of the FY2004 = (100*24 + 33*6 ) / 133 = 19.5
This is what they are going to do exactly and announce that they have exceeded the targets. President Bush will congratulate them and ask them to keep it up and they will keep their game going

. That's why i have raised this question in the latest letter that we are planning to send to the congress men. Look at complaint number 3 in that letter :
http://boards.immigrationportal.com/showthread.php?t=132067