Ah, now I understand the problem.
Your sample movie clip has two frames, so the tween will animate "currentFrame" from zero to one.
When you animate a number from 0 to 1, it will run through the following values:
0.00 - 0.25 - 0.50 - 0.75 - 1.0
Between 0 and 0.999, it will always display frame "0" (because the value is cast to an integer); and then, exactly when it reaches 1.0, it will be removed from the display list -- so you never see the second frame.
Do you see what I mean?
To work against that, just animate between 0 and (numFrames - 0.001). Now it will tween the number between zero and 1.999, and you'll correctly see the last frame.