Subscription Plan - monthly rate counts one extra month

The Subscription Plan doctype, when set to Monthly precide determination adds an extra month.
Is there any reason for this?

For example, if the subscription is for 1 year, it is counting 13 months instead of 12.

This piece of code: erpnext/ at da43a5e371dede52acd26791ffa80ba4a13fc0ef · frappe/erpnext · GitHub