improve cardinality estimate

I recently saw a query with a wrong execution plan and looks to be caused by an overestimate of a particular where clause. Ex:

select

    count(*)
from

    orders
where

    partition_date = sysdate-1 and
    order_procesing_date > = business_procesing_date

the count itself is = 42K
the count itself without second condition ~ 24M the cardinality estimate ~ 24M
the cardinality estimate if I add parallel hint: 1M (this is an interesting one and looks to be related with the fact that parallel queries get DS level 8)
the cardinality estimate if I rewrite the condition : order_procesing_date - business_procesing_date >= interval '0' minute :1.4M

I haven't tried yet but I suspect that if I create extended stats on expression ( order_procesing_date - business_procesing_date) can get me better results

The question is what is the logic behind beter cardinality estimates if I rewrite the where clause?
What will be the best approach for this query?

Thank you.

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