aCalendar is a user interface to the Android calendar storage.
aCalendar has no own data and provides no own synchronization.
Synchronization is always between the cloud server and the Android calendar storage and provided by a calendar sync adapter. For more details on a sync adapter, go to the help page about the specific cloud service.
That is why all normal calendar apps show the same data (only some apps, like the MS Outlook app provide their own proprietary sync and do not integrate into the Android calendar system).
For problems with calendar synchronization see Troubleshooting Cloud Synchronization