The prime minister is named Rogue. Yeah, that tracks. I know that this fantasy setting doesn't exactly have classes and levels, but it's actually quite reasonable if Rogue were the prime minister's class as well as his name.
It makes sense to place the highest level character in charge of something. They are the most experienced, and it's just logical that others will follow them. There is a fun quirk of AD&D rules that helps explain our modern world, and this fantasy setting, as an artefact of this logic.
On its own, and not part of a multiclass combination, being a thief in AD&D kind of sucks. They are barely better than a magic-user in terms of attacks, hit points, or gear allowed. To say that thief skills are unreliable is underselling it; they fail far more often than they succeed until you are already a high level character, at which point spells and magic items have long since outclassed almost everything that you can do. However, a straight-classed thief levels faster than anyone else, even druids (at least most of the time) which means that you'll hit a given level a lot earlier than nearly anyone else.
Paladins, by way of comparison, have the worst XP chart, and level far more slowly than other core classes. They are the ones you actually want to be in charge of things, as they are inherently trustworthy (doing anything that isn't lawful and good revokes their paladinhood) and their incredibly high Charisma helps them convince people to listen to them. But, their slow levelling means that they aren't likely to be the one with the highest level in a given group, and thus not often be placed in charge of anything.
Collectively, these things mean that one will most often see thieves in supervisory roles or those otherwise involving oversight. This is especially true for things that aren't directly related to a particular class's talents. That is, while you probably won't see a lot of non-fighters in charge of anything military, or non-mages overseeing wizarding schools, a position like the president of a company--or in this case, prime minister of a country--is fair game to any class. In other words, the people in charge of companies and politics are likely to be thieves.
It doesn't take much observation of our Real World® to see that this checks out.
Author actually complicated this further than that, on purpose. Just know that Lyle, his brothers, and their father all consider themselves male, there are reasons for the confusion, and even for the family to consider Lyle the failure of the family.
"No son of mine is going to be a woman!" --Lyle's dad