When Harry, Ron and Hermione were first trying to complete their task "strange" things kept happening, such as being hit with rocks, saving Buckbeak (whether he was actually killed the first time, or it was just implied, however the wincing look on the trios face upon witnessing the execution would suggest that he was) howling to distract the transformed Prof. Lupin and casting a physical stag Patronus. 

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> It was later revealed that these were in fact caused by their future selves via use of the Time Turner to try and change how events went down.
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After using the time turner, the "present" Harry and Hermione (as opposed to their future selves) were now the ones committing these acts. The same disruptions seemingly took place, yet the timeline somehow changed.

How is this possible?