I have been modifying my Facebook and Gmail account settings when I come across several types of saving strategies. Take for example, at the time of this writing, Facebook:
Instant save - saves upon modification. Mostly used in Privacy settings. Modifying them immediately applies the selected options.
Item save - saves a bunch of settings for a certain item. An example is the one on General Settings. Editing a Name asks you a bunch of inputs and there's it's save button. Still under general settings, there is Password, Email etc. each having their own save buttons.
Sectional save - one button that saves a group of settings. This was found on the older Timeline, where editing relationships made you input a bunch of people, their data, and one save button for all.
Global save - This one can be found in Gmail settings. Exiting the settings page without save prompts you to either discard or cancel. What I do usually is go back to General settings and save there, regardless of which section had an unsaved modification.
I don't know why websites implement them differently. Shouldn't they be consistent? Are there any guidelines as to how these save schemes should be done?