Dev Admin is meant as Dev manager here, as for normal devs are called simply Developers, since each sub group has a "leader" with an additional "Admin" in the end as in supervisor.
as for the PRs both are present since the main PR manager oversees the PRAs because there's more than 1, they're divided into 3 locations/timezones and in their turn oversee all the PRs designated in their own region.
But in 2018 right now, there's too little amount of developers meaning that there is no need for an Dev Manager to oversee the Devs just for 2 devs, the manager can do that, which is why the spot is Vacant I presume.
here's the link to the current staff list, you'll see how few devs there are and get a better understanding of the situation.
https://www.era-go.com/forum/showthr...ist-(Complete)