I emailed AAMC about this. Here is their reply:
Dear Applicant,
The transmission date indicates the most recent date that a medical school has downloaded information from AMCAS.
Transmission dates prior to your application processed date indicates the medical schools have downloaded preliminary applicant information ( contact information only ) . Transmit dates after your application processed dated indicates the medical school has downloaded your complete, verified application.
Additionally, these dates may change throughout the processing of your application, depending on how many times the medical school downloads information regarding your application.