Your stripe dashboard shows you the number of subscriptions in different plans at the particular instant. These users could have originally been part of another plan and then upgraded or downgraded to a different plan. Stripe dashboard shows the current state of every customer and the plan they are in.
The “Subscriptions by plan” insight captures users who subscribed to the different plans when they first subscribed to your business. If you automatically place all your customers in a free plan and then they subscribe to different plans, you will see all your customers in the free plan. If however you allow your customers to choose the plans they can subscribe to after sign up, those numbers will show up in this insight