I’ll start off by saying that I’m a bare bones beginner so may be wasting your time here – but I do know a fair bit about the N64 ;)

That said I’d guess that it might have to do with the maximum values the controller is sending through for the stick movement. Typically I don’t have access to any documentation right now as am at work, but I think there’s a min and max range value set in the config files which is a half int (32762) and changing that range could be used to tweak the sensitivity issues.

The way to figure out if this IS the problem would be to run jstest and see what kinds of numbers you’re getting out of the controller. Given the bad experiences I’ve had trying to get modern pot sticks to work in the N64 “light gate” system I’d guess the sensitivity would max out at about a third of the pot value, and it’ll also behave a little different too in terms of how quickly the numbers become large.

Hope that helps!