Something very odd is happening!

I'm trying to animate with very basic sprites. I create graphic symbols out of each position, then create a movie clip with each graphic per one frame. But here's the weird part: Although the animation is fine, some of the frames are blurry! If I zoom in to 200%, the pixels are sharp. Yet at 100%, 4 out of 6 frames are a tad blurry.

I have the original sprite image set to Lossless and Allow Smoothing unchecked. What am I doing wrong?