Explanation of texture compression settings?

Any more info about this? I’m working with gradients and hand painted faded alpha’s and it seems like HDRCompressed is the only 1 that doesnt produce banding and line artifacts. Is this normal?

Is this the correct compression to be using for dealing with faded out transparency maps that use custom made gradients? In photoshop there is still a bit of banding visible, because we all know how hard it is to get perfectly smooth gradients, even by hand painting them, at least it seems hard for me so far, although this compression setting seems to smooth it a bit?