This is how Voyage works. Unlike Outpost, that has a "If this is the first time you played an Outpost this turn" clause, Voyage lacks this. If you play X Voyages on a turn you get X extra turns, provided that at the time it was played, the previous turn wasn't yours.
All of the extra turn stuff works this way, but all the others have limits on each source. So if I played an Outpost, bought a Mission and activated Island Folk all on the same turn, I'd get three extra turns as long as the previous turn was not mine at the time each one was originally played / bought / activated.