The reason could be test anxiety which is a reason I've heard before but never bought. This is because I was not anxious during the test. I remember thinking, "This is hard, that's ok though, the test on a curve. Better yet everyone else is taking the same test, I bet they are all panicking and doing worst because of it, great!"
The other reason which sounds like it fits me much better is over confidence. I was excited every time I turned to a new section and found nothing but LSAT arguments looking back at me. No daunting reading passages, no games with tricky rule wording, just lots and lots of logic games that looked delicious to my eyes.
The Logic games can be tricky and you do have to be sharp to get the right answer. Many of the questions are trick answers and they, (like the rest of the test,) are not the time for your brain to take a breather. Reviewing the September test many of the wrong answers I picked were careless mistakes. Things that I really should have gotten right.
This leaves me very pleased because now I have an answer. I'm no longer pulling out my hair crying, "what the f@ck happened?!?!?" This is something I can fix and do better in next time.