Yes, this does happen occasionally. There will be fluctuations from campaign to campaign, and this is valid.

However, what we can see in the correlation between clicks and opens is, on average, there is a strong correlation between opens and clicks and optimizing on opens is valid. Optimizing subject lines on clicks tries to leverage too small of a sample and too noisy of a metric, essentially selecting a winning subject line at random.

You can read about this in more detail here.