MoM retention looks at the rolling 30 days. Anyone who made a purchase, not just new customers. Did they come back in the past 30 days and make another purchase? It includes all customers.
Customer Retention by cohort looks at monthly cohort retention. Customers are put into a monthly cohort when they first make a purchase.
The months running vertically on the left show the month a customer was acquired which was when they made their first purchase. A customer will always be associated with the month they made their first purchase, even if they purchase every nine months. Peel does not have a cut off date after which churned customers are considered to be new: customers only ever belong to a single cohort. As you look to the right horizontally, you see the retention rate so far for each month. It is a number that changes as each month goes by because you are still retaining or churning people as time goes on. The percentages go down only when customers stop purchasing in the future.
If you toggle the percentage at the bottom of the chart, you can see the number of customers instead of the percentages. In column 0, that is the size of the cohort.