It might be something like this, I think. You can check for the most accurate wording in the anime, but I remember that, upon getting his Zenkai, Vegeta performed a huge power up, to which Gohan was at first unimpressed, saying things like "They didn't change that much", to which Kuririn scolded Gohan in a similar way he was scolded by Vegeta before, stating things like "try to sense Vegeta's power right in its depth... he is calm but has an astounding power...". Granted that that scene is filler, but I always found it similar to the way Vegeta scolded Kuririn before, and got the impression Kuririn became more watchful to getting a sense to a person's powers after getting scolded by Vegeta because of it.
Regarding the Nail scene, do you remember what Kuririn exactly stated? It might be that Nail, when not using his full power, was already way beyond from the other non-elite Namekians, and this difference just turned out to be even bigger when he powered up.
Kuririn initially judged final form Freeza by his appearence, stating he doesn't seem so fearsome, then he got corrected by Piccolo, who stated that it's a good proof for why someone shouldn't be judged by his appearences... I would say this might be comparable to this situation, but thinking again, the situations are different ones. In the final form Freeza case Kuririn was just misjudging him based on his softer appearence, whilst in the 3rd form Freeza's case, it seemed that Kuririn couldn't get a true sense to Freeza's powers.
Another thought, but maybe this "latent" power is somewhat akin to Vegeta and Nappa on Earth... they couldn't supress their powers, yet the Z warriors couldn't get to know how dangerous they were until they braced themselves to fight... maybe Vegeta's chi sensing is so sharp that he can sense Freeza's real capabilities even when he is still not fighting, but he can still sense it since he isn't really suppressing his powers, he is just hard to be sensed (like Vegeta and Nappa on Earth) since he still isn't fighting. Kuririn was like sensing the surface of Freeza's powers like the Z warriors were when Vegeta and Nappa arrived on Earth, but Vegeta at that point could get a true reading for someone that isn't fighting but can't suppress, just like a Scouter would. Maybe it makes some sense?