Compression days vs. Late supply pegged to Forecast??

Hello All

I am looking for better clarity and understanding than just the definition of the above exceptions.

Compression Days reads as :  items with planned orders, discrete jobs and purchase orders that have compression days and need to be expedited

Late supply pegged to forecast reads as : items with shortages that prevent forecast demand from being met, caused by component shortages

By these definitions shouldnt I expect items that have the exception message for Late supply pegged to forecast to also show up as items with compression days?

Bottom line our exceptions master query set up years ago never included the Late supply pegged to forecast but did include compression days.  We recently had an item lead time change reduced drastically from an overseas supplier but the planning time fence was never adjusted on the item.  So the only exception message we were getting was for the late supply pegged to forecast which went unnoticed for a couple weeks.  Im trying to understand these exceptions better to figure out why we didnt see a compression days message for the planned orders?