Probably just coincidence. OmniFocus syncs 1 minute after you make a change, and hourly thereafter. Both devices have to sync, and there is no synchronization of the respective schedules, so it might be as much as an hour before the other device does the sync that pulls over the change. It might also be as little as a few seconds.
Now, if you are using the sync server built into the OmniFocus application on the Mac, things are a bit different because the server immediately notifies the app (on the Mac) that a change has come in. But in all cases, the operative model is the same: each device speaks to the central repository, sharing and collecting all changes since the last sync. For a change to go from device A to device B, A must sync, followed by B once A's sync is complete.
It's good practice to tap the sync button when you know you are done using one device, and similarly to tap the sync button when you start using another. You are assured of the freshest data, and it helps keep your database compacted and syncs faster.