I am not a scientist but I am a coffee enthusiast. In that respect I'll give you the best answer I can:

There are several reasons for why coffee out of a vacu-pak may not taste right. Proper tasting coffee comes from arabica beans; though frequently a brand may advertise they are using 100% arabica but may only be using 10% arabica with 90%robusta beans - which taste worse.

Most coffee purchased in a tin is usually very close to being stale. This is because the oils which provide the excellent 'coffee flavor' begin to break down almost immediately when exposed to air. While they are vaccuum sealed the fact that they are pre-ground means they will deteriorate that much faster. Generally once beans are ground they are only 'good' for a few days.

Short version: Basically in my experience they are using cheaper grades of beans which, having been pre ground, have lost almost all of their good flavor.

How to get the best coffee: Have freshly roasted beans, grind them yourself. A bean generally will last a few weeks if roasted and unground so that's another possibility. Locally we use:

http://www.greencastlecoffee.com/

That's the best I've got. Anyone else care to take a crack at his question?