The contents of the sack are not invulnerable, they're just elsewhere. Many attacks and effects do not work across planar boundaries, so they simply cannot reach the contents. Anything that can reach them affects them normally, and that's when the +2 save bonus comes in.
You can think of the sack as containing a special transport gizmo. Anything going into the sack gets teleported to a hidden storage area somewhere else in the universe; when you reach into the sack for a specific item, it is automatically retrieved.
Since the "contents" are stored elsewhere, the sack is empty except for the magic gizmo. That's why it weighs so little. If you carry the thing on your back, you're lugging the gizmo around, but the storage area doesn't move; it stays put, because the transporter gizmo can retrieve its contents from anywhere.
Since the storage area never moves, its contents never feel any impact. You could drop the sack off a cliff, but nothing in the storage area would know the difference. The impact might break the gizmo and stop the transport effect, but it wouldn't harm even the most fragile item in the storage area.
Now, if you open the sack and pour acid into it, the gizmo will treat it like any other incoming object. The acid will be shunted off to the storage area, and anything already there will have to make saving throws. In such a situation, everything inside gets a +2 resistance bonus on its save, by virtue of the storage area's magic.
Do you get it now?