The centre of mass of the ISS follows a fixed orbit round the earth (to first order approximation that is, occasional thrust is required to maintain this trajectory). Moving away from the centre of mass, by definition, puts you into a slightly different orbit. This perturbed orbit has a different trajectory and orbital speed. This will manifest itself as very small reactionary forces relative to the ISS structure, in other words what you and me call gravity. In plain English, except for at the exact centre of mass position within the ISS (Or any point exactly on the trajectory traced by it) objects will appear to drift.

In the microgravity tower, translations within the experimental container will not result in divergent trajectories, so no drift will occur.

I think they're implying that if you want to test to effects of microgravity, you don't want something like 14 psi of air pressure happening at the same time, because interactions with the air flows might confound whatever you're trying to test.

The station is likely better in terms of gravity, but in terms of test conditions, you're somewhat limited by the astronauts being there.

Perhaps the ISS has more vibration from all those life support systems?