Why aren't these queries the same?

I'm going through a process of identifying our biggest I/O hogs and working with the devleopers to fix them. One statement has us stumped. The original looks like this

select count(*)
from scott.table_a
where (trunc(sysdate) - trunc(scott.table_a.mod_date) >= 12)
/

so what they are looking for are instances of mod_date that are 12 or more days old.

We re-wrote the query to look like this

select count(*)
from scott.table_a
where mod_date < (sysdate - 12)
/

but they don't return the same results.

I did confirm that trunc(sysdate) returned the same value as sysdate and trunc(scott.table_a.mod_date) returned the same value as scott.table_a.mod_date.

--
Ed Stevens
(Opinions expressed do not necessarily represent those of my employer.)