The problem is with EditDate of opportunity where time is not always updated. If you add additional contacts or remove existing contacts from the same opportunity without changing anything else, time in EditDate will not be updated.
Here are the steps to replicate this scenario:
1. Create opportunity in ACT! for today and in Schedule With field add for example, 1 contact.
2. Save the opportunity and check EditDate
3. Now, open the same opportunity and add 1 contact more and remove 1 contact (don’t make any other changes)
4. Save the opportunity and check EditDate. EditDate will be the same as in step 2.
I tested this using ACT! 2010 Because of this, I'm unable to detect the most recent changes of opportunites, if the changes are only related with adding/removing multiple contacts.