Not sure when this was included with stock cart. I don't think it was with the newest release.
There is a field in products table
protected $attrSepTab = true;
which controls if specification tab shows as a separate tab (true value) or its content shows under description tab (false value). There is no way to control this behavior from admin - at least I did not find it anywhere. This field and its value can be changed with import or directly in database only.
However it has been argued few times already that attributes showing under description - weight, upc, sku - should not be there to begin with. None of these 3 fields is part of the description. If they show at all they should be under Specifications tab. So here we are with completely reversed behavior - instead of moving these fields under Spects tab XC decided to move Spects info under Description tab....
Why? It makes no sense at all...
And why there is no control in admin for this field?