In a nutshell, I scored a 153 in June, cancel in Sept, and a 164 in December. Should I include an LSAT Addendum in my applications to explain this discrepancy or let the scores speak for themselves? Any advice would be greatly appreciated.

If you have a really good reason for low score and cancelation then do it, if not then don't. Good reasons would include something like serious family problems, sickness etc.