I havent held mine yet but think of it this way. When collapsed the "tube" for the lack of a better word is all the way through the stock fully supporting it.
When open only part of the "tube" is supporting the stock. M4 stocks are the same way.
Just and opinion
tacman605 has it right. As the stock is extended out, there is less surface area from the tube making contact for support. My Classic and HOLO both feel a little sloppy fully extended - that's true for any gun on the market.