[piggity]

Because your sample is so small, the results are easily skewed by a few people. As the sample size increases you'll get a more representative picture of what is happening.

It doesn't invalidate what you've found, but does mean you should now continue on with your experiment to increase the sample size - and help ensure your data wasn't impacted by something else - e.g. vacations / a conference that was attended by many of your subscribers / etc etc.