Unfortunately, testing this myself, in my own instance, this and a lot of events came over. You may wish to check if there are any timeout errors on your server or other issues.
Additionally, we did make some changes in regards to things for changing the Calendar application over to Events in 4.7.3 so that may also have helped me import this. If you continue to see this issue on 4.7.3, I would advise checking your server for timeouts, errors, etc... If you see anything software related, please let us know.