At least, it makes sense to me...
When it comes to adventurers, there are two primary groups: Fighters and Mages.
The average adventurer's highest stat is usually between 500 and 700 before they level up. When it comes to stats that aren't magic, mages are usually well below this 500 to 700 range.
As it stands, the physical combat stats of mages is comparable to the stats of average fighters who are one level below them. This allows for a fighter of one level lower to defeat a mage as long as the mage isn't allowed to use magic. The simplest example of this is that Lefiya had to be level 3 before she could fight a minotaur despite it being a level 2 monster.
Now compare Bell to the average (or even slightly above average) adventurer. The average adventurer's stats basically makes them look like a mage when compared to Bell's stats.
The comparisons go as follows: a mage (without using magic) of level (x) is comparable to an average adventurer of level (x-1) which is comparable to Bell of level (x-2).
This rule wouldn't apply to abnormal adventurers.
As I see it, a max level 6 Bell will not stand a chance against the current Ottar because Bell's stats were never much better than Ottar's stats to begin with. Only in agility would Bell be competitive. I see Bell having to be level 7 before he can hope to best the King.
The current level 4 Bell would lose against a mage like Riveria because she actually has good stats for a mage.
*These comparisons rely on the premise that Ottar and Riveria had somewhat similar stats as they have now for their previous level ups.
Mathematically speaking, this effect would only get more severe at higher levels. The weak get comparatively weaker while the strong get comparatively stronger. But that requires a more nuanced look into the stats than what I'm talking about here.
As a final aside, I am also implying that Ottar or someone like him could also overcome a level gap given the right situation. I don't see this being limited to Bell. It would just be exceedingly rare for a multitude of reasons.