It mean tops slide 'up' all the time. I even noticed my (non knit) RTW clothes did it as well.
So first I took the pattern for a top I wanted to make (the portrait blouse from GNBFBS) and did my trial and error method. See the long dart in image. Problem was it wasn't accurate enough.
So I looked up the 'proper' way of doing it. Traced the existing dart and pivoted it up to the place I'd found using trial and error method. Taped it in place.
Tried it out and tissue fitted. Seemed ok to me.
Next step is to make a wearable mock up of the blouse.
On a side note I did raise the neckline and will be adding a peter pan collar and sleeves (added about 1.5cm to the armhole- hope that's right).