To be a paying subscriber, keep in mind that a customer must have paid an >$0 invoice. $0 subscriptions and $0 invoices will not be included in this report.
To be counted toward Churn zero, a customer must not complete one full month as an active subscriber to be included in month zero. They must terminate before that month has completed.
Now let's look at the following scenario as an example:
We'll say that a customer subscribes to a plan on November 1st AND pays their >$0 subscription invoice. Later, on November 20th, they decide to terminate their subscription, 10 days before their subscription was set to renew (on December 1st).
Since they did not complete one full month since their day of signing up - that merchant would be considered a Paying Subscriber AND churn for month zero.