Finches, collectable shades are noticeably different from each other when adjacent. Rarer shades are established when there is evidence that sheets of that shade (sometimes very few) were produced at some point.
The stamps on your block have the illusion of being different due to the heavier postmark on the top right stamp.
It is possible to have differences in ink intensity on a sheet due to hiccups in the printing process but not in this case.
I have transposed the middle sections of the upper right and lower left stamps of your block so that the consistency of shade can be seen.
