Why do some Live Photos play in fast-forward-type speed? (Regardless of finger pressure)

I have had the iPhone 6s with iOS 9.0.2 for a couple of days now. I have been using the Live Photo feature quite a bit but I have noticed that some Live Photos play back as if being fast-forwarded. This is not a result of increased pressure as I have eliminated that possibility.

I wonder is it resulting from the low frame rate used or is it simply a device fault. It doesn't happen every Live Photo however so I'm stumped.

Any help or info would be really appreciated!