Three Different Perforation Types

I have always found it interesting in how three different perforation types are found in this set of U.S. "American Arts Issue" stamps.
They are as follows:
Scott # 1553 (perforation 10.5 x 11, Electric Eye),
Scott # 1554 (perforation 11.1, Androetti),
Scott # 1555 (perforation 10.9, L-Perforation).

Other sets:
"Performing Arts Issue" Scott # 1755 (11.1, Androetti) and Scott # 1756 (10.9 L-Perforation).
"Christmas Issue" Scott # 1799 (11.1, Androetti) and Scott # 1800 (11 x 10.5, Electric Eye).
"James Hoban" Scott # 1935 (10.9, L-Perforation) and Scott # 1936 (11.1, Androetti).
"Christmas" Scott # 2063 (10.9, L-Perforation) and Scott # 2064 (11.1, Androetti).

Yes, this sometimes happens in sets as well as sometimes on the same issue i.e.John Paul Jones Scott 1789 and 1789A.

Here is an interesting website that discusses this further and also has a list of many similar looking stamps with different perforation types. The ones that I listed above are different looking stamps in the same set that have different perforation types. I made this smaller list because the other list has already been made as follows: http://www.ebay.com/gds/Guide-to-Pe...45442/g.html