PUSH BUTTON TRANY QUESTION

Working on my dads 62 polara w/ a push button. We installed the shifter cable & that's all we got done before he passed. He knew more about these than I do. The trany pan was off. With the neutral button in I adjusted the cable so it was on top of the neutral safety switch. I pushed every gear & checked every time to make sure that it was in that gear. Also made sure that it would not start in any gear but neutral. Ok put the pan back on, put trany fluid in. But before that I made sure it was in neutral again. Started it up & noticed that the speedometer was moving. Looked underneath & the trany was turning. (drive shaft is off) If the trany is in neutral why is it turning ? Now the trany has a reverse manual valve body. Is that the difference ? What am I missing