One of sections inside Keyword Surfer is Similar Keywords section.
It generates around 10 suggestions for keywords which has similar SERPs and list them with search volumes for your location.
Similarity is calculated based on top 20 pages from SERPs. If "1 week diet" has 10% similarity, it means that 2 out of 20 pages from its SERP are the same for the primary keyword or a related term.
For wide keywords, Surfer usually generates dozens of suggestions for similar keywords. If you want to get more of them, you can use Keyword Research tool (for subscribers only).