It totally makes sense for the amount of attempts to vary from NPC to NPC. (Some are resolute in their actions, others are on the fence, etc.).
I think that might be difficult to implement in the game though (As it would need to be set for each NPC). So, a scaling DC each attempt probably would be easier to code.
I mean, they have to manually set the dialogue for each NPC, so it shouldn't require that much extra work to set a "num_attemps" flag for how many checks are allowed.
Larian also could just reserve this for the important checks. Kagha, Nettie, Hag, Minthara, etc. Nettie seems more gullible, so 4 checks could be allowed (which is basically already what happens, just with only one character making the checks). Whereas Minthara and Kagha are more stubborn/suspicious, so you'd only get one or two attempts.
