It's difficult to know exactly where certain firms' program trades will kick in because for some indeces - such as the Nasdaq 100 - the percentage ratio of a certain stock in the index changes every day by a factor called the DRM (Depository Receipt Multiplier).
For this reason, today, three issues ABCD, EFGH, and IJKL might be weighted 12.5%, 7.6%, and 5.3% in an index today; tomorrow they might be weighted 12%, 7.9%, and 4.9%. While seemingly miniscule, these changes may account for hundreds of thousands of shares.
As we all know, issues vary in liquidity; a change on the order of a couple of hundred thousand shares of a certain issue may affect that liquidity, at times dramatically. If a number of program trading firms are looking at roughly the same program trading strategies, obviously lots of shops are trying to grab/pour the same stocks. For this reason, some firms may avoid goosing certain highly sought after, illiquid index consituents by keeping them in inventory, trading them from there.
So, between the daily DRM fluctuations, the varying open market purchase and sale requirements of certain firms required to perform index arbitrage (on the cash end), and different strategies/"recipies," you will find that the numbers aren't exactly flush.
Rather, most professionals focus upon "price zones" wherein program trading activity is likely to occur.
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