This post fully answered my question about, "why I need to purchase the app separately for different platforms".
I believe this is an appropriate place to leave some user POV feedback about why I am chill with purchasing for separate platforms- but if not feel free to delete:]
I am not typically a person who spends money on apps or subscriptions. I've been using and really enjoying workinghoursapp since mid May this year. I purchased pro right before the trial period ended. I can't currently pin down exactly what has made this app work so well for me when compared to other similar apps... While the app isn't totally perfect, my experience using it these past few months outweighs my aversion to paying for things. So I will purchase for separate platforms.
I think if I had originally subscribed rather than fully purchased Pro I would feel differently- but the subscription service seems to be a middleground for those still unsure if they want to purchase Pro after the trial. I think it is good to have options. Cheers, Ariel