The usual way to tape a typical butt drywall joint is to put .5 cm of drywall compound over the joint to create a bed of "mud", then add tape, press in to get excess "mud" out, then apply more "mud" over the top of everything.
This all-purpose drywall compound shrinks as it dries because the water evaporates out of it. Does this evaporative shrinking introduce voids under the tape?
This seems especially to be the case for factory edges because they're so significantly tapered that a good deal of drywall compound is required to fill the space. This means there will be a good deal of shrinking if, for instance, drywall compound loses 15% of its volume to evaporation (just an off-the-cuff guess). Tighter joints (cut so that they fit very tightly) won't suffer the same amount of shrinkage (by volume) though there are other problems with not having a good amount of drywall compound in the joint.
Best Answer
Under the tape, you should have no voids. If you do, you likely didn't press it into the bed of mud well enough, or you didn't have enough mud period. The mud acts like glue to the tape (properly applied).
Mud is designed for multiple coats, however. Even if the mud pulls the tape in some, a subsequent coat should fill the gap.