I don't know where you pasted the data from, but the first JR ferry of the day leaves Miyajima-guchi at 6:25. And the last JR ferry in the opposite direction leaves Miyajima at 22:14.
What does those little prints below the table says?
1) on busy weekends and national holidays, ferries operate every 10 minutes between 9:00 and 17:30.
2) All ferries can also carry vehicles
3) Duration of the ride is 10 minutes
4) Ferries departing Miyajima-guchi between 9:10 and 16:10 will make a small detour to pass close by the large torii gate, but the detour has no impact on travel duration.