Indeed, it is very close distance. And this should give a little DOF. I see "ok" the area i have tried focused in most pictures. But in 2 images I see the text a little blurry or miss-focused. Maybe the white-green border you say could be a better target for the AF.
What I cannot understand is that if between frames i didn't move the objects neither the camera and the conditions are the same in all pictures, why is the focus changing? a little, but changing. AF module calculations should not be like mathematics in the same conditions (always giving same calculated value for the same input values)? It should not be like 2+2=4 always?
I'm sorry about these extensive explanations and questions, but i am very frustrated about this. These are only tests, but in real situations although most of times it works ok, sometimes the focus goes to the back or to the front unexpectedly, ruining the picture.
And what I don't know is if the camera AF is damaged or if i am doing something bad or if the camera can not give more.
Maybe I send the camera to the service and it returns with a message like "AF was working ok". Maybe I don't send it and it is damaged and I am not giving all a D300 is capable of....
Thank you for your help JosephK. Like you, I hope someone else can give a bit of light to this.