err... there are a lot of fine details here\left and \right do two things. First they put the enclosed content into an atom of the type "Inner". Secondly, the height of the parentheses will be automatically adjusted according to the enclosed sub-formula. The following table may help you. It appears on pg.170 of "The TeXbook". 0,1,2,3 stand for no space, thin , medium and thick space. $a+(b+c)$ is decomposed to atom sequence: ord 2 bin 2 open 0 ord 2 bin 2 ord 0 close $a+\left(b+c\left)$ would be: ord 2 bin 2 inner It is then natural to have difference in spacing in some cases. (more...)