Hello,
There are 3 ways you can connect to and access other accounts:
1) "Development" connections, which exist for testing and only give you testmode access to another account
2) "Production" connections in livemode, which is the common use case
3) "Production" connections in testmode, which some platforms use to enable their customers to test things. For example, if you run a store and want to try out how the store interacts with Stripe, they can let you run your store in testmode.
From the platform's perspective, #2 and #3 are both "production" features: it's a capability built into the platform for its customers. That's why we include both in webhooks. Also because it's very simple to filter out, as long as you are made aware of its existence.
Thanks,
Brian