Weirdness of _currentframe property

When I had put onEnterFrame function in a movieclip to debug where the current timeline is…

and the trace kinda looked so odd…

trace was like this…

>> mc1 : frame = 1
>> mc1 : frame = 1
>> mc1 : frame = 26

and I saw no sign of jerky animation between 26th and first frame… as a matter of fact frame that I was seeing was 26th…

sorry cant post the actual source will try to reproduce in a separate ex… meanwhile if anybody has any idea ?