Author Chapter 6 (Tensei Shite Inaka de Slowlife wo Okuritai)
> The "liutenants, ministers and magisters" will by definition have power
Sorry, I meant not as much power nor protection as a regular lord - they can only act as representatives at the most. They don't own the territories they manage.
Perhaps there might be a situation where a lord would have been tasked to manage a territory of the king, but that territory is still the king's and not the lord's fief.

Delegation can happen regardless of vassalage. Mere governors are still distinct from lords as lords take ownership; governors are appointed to govern a land, lords are given land. If the king could help it, he would rather keep all the fiefs for himself, and then appoint governors/ministers/magistrates to manage his lands and collect tax. Alas, the feudal ties are older than he is, and he has to keep his family's vassals happy.

and btw, I wrote "magistrates" and not "magisters" in case you misread that.

Right, differences everywhere. I would guess that this isekai's kingdom is one where the lords are practically powerless, or the king is extremely powerful - almost as if it was an empire.

I think you are going at this from the asian perspective, where emperor had absolute power and important people were given offices or land to govern for him. What is shown in this manga is your typical European feudal system, where land is given to the person. Centralized rule vs decentralized. King owns only Crown Lands (often including major cities), which are his personal property. It is also main source of income for the kingdom treasure, from which he pays his private soldiers and all different other things that he needs. Rest of the kingdom is property of his "vassals" which means people that gave promise of vassalage and in return were given the land they own by the king. They have to recognize king as their master, serve and protect him and his family, but they are powerful enough in their own way, sometimes even more than the king. But it wasn't that often they revolted for 2 reasons :
1. They had to break their official pledge, and everyone would know that
2. Other nobles would come and help the king