Returning Rate vs. Repurchase Rate

The answer is - one is a non cumulative metric and one is a cumulative metric.

Returning Customers by Cohort vs Repurchase Rate Report

Returning Customers by Cohort is not a cumulative graph.
It tells you the % of customers from that cohort who made subsequent purchases. For example, the October 2020 cohort - 20.5% of the customers who first purchased in October made subsequent purchases in the send month (November) , while 14.1% made subsequent purchases first after they purchased in October. It is looking at the activity in each month after the initial purchase. This helps you know when to remarked to customers to get them to come back and buy again.

Repurchase Rate is cumulative, so it tells you when and how much of each cohort has repurchased and the number grows as time goes on. Obviously the earlier that people come back is better, and repurchase rate shows you when and at what percent that happens for the cohorts or in average. So for the October 2020 cohort, by month 1, 14.1% came back, but then by month 2 (November) 28.95% of the cohort has purchased two or more times. It is cumulative, it includes the 14.1% from the first month. Then by month 3, the cumulative total shows 32.16% purchased.

Returning Customers is showing you the amount each month individually - not cumulatively.