Hi BB42. I'll try to shed some light. The Endura dnd Delica models are the current workhorse models of the Spyderco line.
The FRN versions are saving some cost in handle construction, so we try to get the best possible steels in those models. This gives you a great deal of performance in a light reliable package. The ATS-55 was replaced with VG-10 in the same way that the ATS-55 replaced the GIN-1 and AUS-8 steels. The constant desire to improve.
The costs of the classic stainless handle Clipits with the best possible steels were getting pretty expensive. The steel handled Endura and Delcia models were offered as less expensive versions of the more classic stainless Clipits. They began as a lower cost item and they began with and still use AUS-6, which is a far more affordable steel.
The Stainless handle Native is made more on the line with the Police model, very good steel and stainless handle.
hope that helps.
sal