The conventional Nwell is shallow. So if there is any noise injected into P-substrate, it's MORE easy to bypass the conventional N-well guard ring. But the depth of deep-Nwell is larger, so for the noise injected into P-substrate, it's more difficult to bypass the Deep-Nwell guard ring. Because noise need to travel longer path to cross Deep-Nwell to affect sensitive ckt. So usually Deep_Nwell is used to isolate the noisy digital ckt from sensitive analog ckt.