They are sort of saying the same thing, both in different semi-deceptive ways lol. -6dB is at least more "honest" in that it is telling you there is almost no sound at that point, but it is really just to make those numbers look better than they are since most don't know what 6dB equates too. Along the same lines, +/- 3 is a way to make it look better than it is, when the swing is really 6dB and likely does actually mean those are the -6dB points. +/- means it remains within that range; aka you should not have swings of more than that between the specified range, 3dB is quite audible, while knowing the -6dB points doesn't tell you if the rest of the range may have larger dips/peaks.