i always wondered about this, may be a repeated thread but nonetheless a question;
why is it that some knives of the same model but of different handle materials have different steel materials? ie Dragonfly FRN -> AUS8 Dragonfly SS --> ATS55?
What Spyderco does is they will keep a certain model going for years and then use different blade steels that happen to be popular at the time. For instance the Police model: When it first hit the scene many years ago Spyderco was making most of it models at the time with GIN-1 also known as G-2. As time went on G-2 was replaced with ATS-34 and later it was replaced with ATS-55 and now the newest and most recent Police model sports VG-10. But keep in mind if Spyderco finds a new blade steel that will outshine VG-10 ( I do believe it will be a while though) then you will see the Police be made with the blade steel of the new era.
Spyderco is constantly striving to improve it's quality. All you just described was older and newer models of that particular model.