You’re referring to your Apple ID Account. It’s not Apple Pay. The billing cycle varies because of the number of days in the months (and leap year) varies. This is normal with any billing cycle. You can see when the due date for the payment is when by following this path;
iPhone > Settings app > Apple ID (your name at top of screen) > Subscriptions > review active subscriptions, billing generally takes place within 24 hours, but network disruptions and declined transaction can affect the actual withdrawal date and time.
In my experience of having subscriptions with Apple for well over 15 years, I’ve never had a charge on my Apple ID Account more than 24 hours off the subscription date, and I’ve never had a payment charged early. You may want to discuss your Apple ID Account with an Apple Specialist. Please review the Apple Support article I have linked below.
Subscriptions and Billing - Official Apple Support