What is the difference between Interval and ThrottleInterval in launchd.plist
The difference between StartInterval and ThrottleInterval is that StartInterval means that the program should be started after x seconds (more or less precisely), whereas ThrottleInterval means that the program should NOT be started again less than x seconds after it was last started.

So StartInterval is used to schedule something to run every X seconds, where ThrottleInterval is used when you schedule something to run based on other criteria, but don't want it to run too often.

"Interval" does not seem to be a valid key - it is not in the man page, as you describe, but nor is it in the source code released by Apple. Either it is a mistype for "StartInterval", or it seems Apple has included "Interval" as an alias for "StartInterval" in their own builds.