Is there a specific reason why we find
max-width:97.5% instead of
100% in common themes such as Twenty Eleven?
Since the TwentyEleven theme also includes some default padding and a border for (fluid) images (see CSS below), a width of 100% would push the image outside of its container's width. This is because of how the box model works: border and padding are added to the width/height.
A better solution would be to change the box model calculation for images in this case. A CSS3 property called