Insiders, in a very large issue such as WMT – which has 3.12 billion shares outstanding- is a Dow 30 component-and is held in countless funds and portfolios as well as having massive trader interest, must be careful as to not acquire to many shares when it is advanced or declined, which they will then have to sharply advance the issue to sell out the inventor or re short and pull prices back and re accumulate before advancing. They can bypass much of these “problems”, with arranged (gap) openings and large movements after the market closes (earnings, acquisitions, etc).