Scott 517c 50 Cent Flate Plate Printing Perf 11 10 On Top

Lori, if you place a straightedge along the right side frame lines in your last picture you will see they line up fine
Now, do that to the left side. It's way off square. So, the picture you took of your stamp is skewed such that the perfs will not possibly be comparable.

Lori ... your post shows a PHOTO of your stamp. It was taken at a slight angle, making the top edge slightly narrower than the bottom edge. You can measure the stamp edges in your photo ... I think you'll find a 1 or 2 mm difference between the two. When you try to compare the top & bottom photo edges, you are comparing edges which are of different SCALEs.
As mentioned in an earlier post, it is best to use SCANs as opposed to PHOTOs. Even then, if comparing two different stamps, one should use a single scan of both stamps as separate scans can likewise introduce different scale factors.
I would also suspect the difficulty in comparing this stamp to a known perf 11 stamp likely has to do with the poor quality of the perfs on your stamp. In addition, you should be aware that perf 11 for one stamp may not be exactly the same as perf 11 for a different design. Scott frequently rounds the perf measurements to the nearest 0.25. So one stamp declared in Scott as perf 11 could actually be 10.75 while another could be 11.25. When making a comparison, it is always best to use a stamp with both the same perf measurement and the same design (or at least one from the same set).

So much easier to just use a perf gauge. This scan effort is like starting a campfire with a bow drill when you have matches in your pocket.

I first carefully de-skewed your photo, THEN brought images together.

Bottom lines: The importance of scanning over photographs.
The importance of mastering the use of a perforation gauge.