In step 4, we selected the top 8,000 patterns (instead of the top 4,000), distinguishing the pattern "word 1 P word 2 " from the pattern "word 2 P word 1 " (instead of considering them equivalent).
a pattern P with a high frequency is likely to appear in two columns, in both possible orders, but a lower frequency pattern might appear in only one column, in only one possible order.