Why are (Pages for Mac) templates, in the Library folder different than what's available in Pages?
The pre-installed templates are stored within the application bundle. You find them by right-clicking the Pages application in the Applications folder and choosing 'Show Package Contents…', then navigating into the subfolder Contents—>Resources—>Templates.

The 'MyTemplates' folder is meant as a standard folder for templates you create yourself. 

(based on Pages 4.3)