Frustrating work situation

[Clowance]

Where I work 3 identical roles were up for grabs. Each involves the main job, plus X, Y, Z tasks, a couple of late nights a week, plus weekends on rotation.
Two positions were filled by applicants but they couldn't find a third person.
Now the boss has approached someone to fill the third position and agreed that they won't have to work any late nights and can work M - F only. They will get the advertised wage, so the same as the other two but with a third less work to do.

I know this is up to the bosses, but had this been the advertised role there would have been lots of people interested in the opportunity to step up and progress their careers, me included. It's a pretty perfect role. All it's done is create bad feeling within the team and highlighted the fact that the boss has favourites.

Just having a moan. And yes I'm jealous this person got lucky. I will grumble for a bit then focus on working on my own job progression

Will they really do a third less work though, or just work more favourable hours?

You're right it's not less work, that was poor phrasing.
They won't have to learn and complete the additional XYZ tasks each day, so not do anything extra than the job they already do. So yes it's more money, more favourable hours, but for the job they were originally doing. Nothing like the advertised role.

I don't really understand the situation, would they be a supervisor or key holder or suchlike?