odds are most of us will not mimic the index's regardless .
we all buy in different times , we rebalance at different times , we sell at different times , we have different tax structures and even index funds vary in expenses and their investment methods .
many are not pure index and loan out stocks or swap minor issues so they don't need to own every stock .
this is an old list of differences in some funds , but you get the idea as to the fact there is always tracking error and differences . the purest is spy .