Calculating from those numbers, I've calculated a P value of 0.003, that's pretty statistically significant.

You'll need to make sure the only variable is the content.

If you've run the ad at different times, or to different audiences, you won't be able to find out why it was definitely, but you'll have a few ideas why it did so.

If you've run it as the image is the only variable, you have two possibilities:
1. You aren't optimising for website clicks
2. You've run that ad for a short period of time, and Facebook's only just optimised for one ad

Otherwise, any changes you've recently made to the audience would also throw this off, one ad creative can be cheaper for one audience but more expensive to another.

Hope that helps
Oliver English