Namu wasn't that strong?

[SSJ2]

At least according to the Daizenshuu:

But it begs the question, why wouldn't Goku be capable of one shotting Namu if he was greatly below Goku/Roshi's level?

 

[DBZAOTA482]

The Daizenshuu is bull in many areas. Power scaling being one of them.

 

[Captain Cadaver]

I wouldn't say the entry outright shows Goku and Chun were a league above the rest of the competition, only that their fight was which is true when it was a constant back and forth of countering each other's techniques as well as Goku's Oozaru transformation bringing it up to a new scale, not to mention being by far the closest match in any of the Budokai. By comparison, Goku VS Namu was very short and Namu was so lacking in skill/fighting intelligence he would've lost to Goku's very flawed tornado attack.

The part about Goku being stronger than Roshi anticipated isn't really any different from what we get of Roshi's reaction after Goku beat Namu.