May I suggest that you dip the stamp face down in watermark fluid. I'm wondering if the stamp has been "repaired". To me, it looks like the possibility of a repaired tear or something from the left edge going through the "U" and just into the inner oval. Also, 2 anomalies at the top:
(1) through the frame line just above the "E" in STATES";
(2) from the first semi-circle perf hole at top right, going diagonally down left and tangent to the small upper right circle, and then extending diagonally through the upper right triangle (notice how the horizontal lines above the triangle appear to shift suddenly).
But this is just guesswork based on the medium-res picture provided in the original post. Some of what I see may simply be artifacts of the from the limited resolution of the picture.
EDIT: I have posted a picture below with red lines marking the starting points of the suspect areas for you to examine.

If these problems exist, I don't think you measure the perforation incorrectly. I think the stamp has been altered, either through damage from natural causes or intentionally.