Maybe you have the stick that is designed to work with the extension? It would be longer.

 

Well, I figured out what the problem was.

The problem is I am an idiot.

As many times as I checked it, I never looked at the backside of the dipstick until tonight, and lo and behold, the back side shows the correct level. The oil level is literally different from one side to the other, depending on which way it is inserted. I don't know what the odds are that I inserted the stick in the same way, every time, but flipping it around, there is an easy almost 2" difference on the oil level between one side and the other. Rotate the stick, and derp, it reads correctly. I can only assume there is a bend in the tube that makes the stick go in at an angle or something. Mea culpa. FWIW, Bob McKormick at Quirk Ford in Mass. (where I bought the replacement part) has been a huge and very patient help troubleshooting this. Over 40 years of working on cars, I never ran into this before. Hope this thread helps someone else out! Thanks for those who posted advice.