What is SERP Similarity?

SERP similarity is a percentage of identical search results for a keyword comparing to the analyzed search term.

The higher SERP similarity the easier it will be to rank for both keywords with the same URL. It is calculated based on the top 20 results. 60% similarity means that 12 of 20 URLs in search results are identical.

If half of the competitors managed to rank for both keywords then why you can't?