I not sure where I found this, but the information is mostly straight on. Tissue containment is the key for comfort and support. Most inexpensive sport bras flatten breast tissue. Mashing the tissue into a uni-boob. Without cup size separation the uni-boob moves up and down freely in your bra. Layering only squashes the tissue more. Though it may not move, mashing and squashing is probably not that healthy for your tissues.
The theory then would be to divide and conquer to minimize the bounce for healthy breasts.
If your sports fit does not look like the one below it's time to find a better way.