They just ported the first game to the Turbo last, that's all. Why? I have no idea. Maybe at the time they wanted to start porting the games they decided Valis II was better than Valis I and went with that instead. Then, when the series proved popular, they went back and ported the first game. I really don't know. Didn't the Valis series start on the MSX or something?
(And in case you're still in the dark-- yes, that last game is Valis I not Valis V.)