It is interesting. At first I thought the single stamp appeard to have darkened or toned or is a different paper? The blue and the green are much lighter and the dark green branches have almost disappeared in the block of stamps. But alas I think I see a line in the selvedge on the paper of the block and the stamps are lighter on the right side. Light exposure?
I would be more inclined to examine the paper of the lower left stamp. Knowing nothing about the issue, I would ask whether or not the issue was printed on different papers and ink was taken up differently in each case. They certainly look different with one paper being much brighter and whiter with the lines of the design being lighter and crisper. That is most evident in the numeral 6. Otherwise, just a standard variation with ink saturation during printing.