If you're analyzing your Plans dashboards and notice that the historical figures for Subscriptions by Plans keep changing, this is expected behavior. Here's why:
Changes in the metrics for past months can be explained by accounts that signed up under one plan, and then switched to another plan recently. For example, let's say a customer subscribes in January 2019 under the Gold Plan and then, in January 2020, changes to the Silver Plan. If you look at the Subscriber by Plan metric for the Silver plan for January 2019 (but it's now Jan 2020) you'll notice that the total has now increased, and the total for the Gold Plan for January 2019 has decreased.
This report doesn’t actually look at the historical plan a subscription was on - it looks at the plan the subscription is currently on. So the total you're seeing for a specific plan in a month's timeframe means that XXX subs signed up during that month and are currently as of today on X plan.