The Oz score takes into account 2 factors:
1.) Amount of competitive content
2.) Overall search volume for semantically-related keyword terms.
Topic A may have 1000 unique searches per month, but 99 pieces of content already out there on the topic.
Therefore, if you create 1 more piece of content, your share of monthly unique users are 10 (1000 uniques/ 100 pieces of content.)
Topic B may have only 180 unique searches per month, but only 2 pieces of competitive content already out there.
For topic B, your share of monthly unique users are 60 (180 uniques/ 3 pieces of content).
Therefore, given other factors are constant, Topic B will outperform Topic A by 600%.