sh_ and ch_ compose the same words

[DONJCH]

While I understand the usefulness of percentages in removing the spurious correlation induced by varying page-lengths, I second Torsten's question. I am also interested in understanding what can be the meaning of overall correlation being greater than that of the individual sections.

The reason overall correlation is greater is that the line is defined mostly by the extreme points from the biological section.
Torsten is correct in stating that the calculation of R assumes that the data is normally distributed. Of course that rarely happens which is one reason why this is not a good statistic to calculate in these very non-normal circumstances.