Sigh. Prim breast textures intended to work with matching tops have the same problem as texture "clothing" in general: they're tied to the specific configuration the designer used when creating the textures.
This is a problem, because one of the wonderful features of prim breasts is that you can resize them. (And these days, it's often via a HUD rather than by editing.) Go far enough from the size the designer used when creating the texture that goes with the top, and it doesn't go with the top any more. This happened to me today with a lovely top I bought. I adjusted to the size I wanted... and noticed a jump discontinuity in the collar. Oops.
What can be done about this? To have any chance of working, one would have to be able to modify the texture to suit the size and position of the prim breasts. Not an easy task, but I've made a similar proposal in the past on the SL JIRA, VWR-10839, and it would have to be something of that sort.