How to explain the flattening of the SRAC curve?

I discovered that there is a way that Short-run average cost curve could become ‘flatter’ instead of shifting. Yet I cannot find an explanation of why and how it can become flatter.

For example, in the picture above, the SRAC4 curve is more ‘flat’ than the other ones. Why is this?

I already looked at Economics by John Sloman, but could not find anything.