I feel the same way. Sometimes I feel like I must have no instincts because none of them seem to fit me very well. I've wondered as well if there might be a fourth instinct that hasn't been thought of yet.
But I think if the instincts were made broader, it might make it harder to determine your variant because there would be so many more factors to consider, and that way no instinct would fit someone completely. It could end up where Sp = this, or this, or that, or some other thing, and you might only fit one or two of those but fit them really well, while you kinda sorta maybe fit all the parts of another instinct, so how do you tell which would be your dominant then? Well, it's actually already like that to some extent, but I think making them broader might make it worse, or at least it would for me. I feel like they're too broad already.
One thing that could maybe work is to split them up more, making more instincts that are narrower, or else have variations of the instincts. For example, there might be multiple variants of the social/navigation instinct: navigation (maps), social climber (status), social critic, follower, etc. That way, you could pick the variant of each instinct that fits you best to use in your stacking, and that could solve the issue mentioned in the previous paragraph. But then you might end up with another problem when more than one variant of the instinct fits you strongly, so I'm not sure how much this would help. I guess there are drawbacks either way, whether you expand or shrink the scope of the instincts.