So while the result makes sense to me (it's what I would have expected) it seems like a multivariable test. I'm not sure I understand why you changed the wording of the title and just had them both be called Read more?
It certainly could be possible that "Read more" is just more inviting then "read next" because they think of it as the continuation of what they're already doing "more" rather then the "next" thing after they've finished what they came for. That said, my guess is that the change that is more important is that 3 images (under read more) offers more choice and so more (total) click through then the 1 image option. They are more likely to see something they are interested in :) (of course too many choices inevitably starts driving down overall click through but I think it makes sense that 3 is not at that point yet).