Different size between the drawing sprite and real picture

Hi everyone,

Recently I am using cocos2d-x to build my first game, but I met one problem. That is the size between the drawing sprite and the real size are different.

For example, if I used an png file that is 26 pixel in width without any margin and drew it as CCSprite in the game. And then take a screen shot, check it in PhotoShop. I found normally it is not 26 pixel any more, maybe 27 pixel or 28 pixel.

Can anyone help me? Thanks in advance.