Netflix is logging your whole activity. Meaning IP-address / device and the time of course. I don't know the exact data which Netflix is aquiring from your device, but it's not difficult to get your hardware ID you are using when logging in with your mobile device. So, while your IP-address is changing, your hardware ID stays the same. Thus, they are aware of you being the user of that mobile device.
My IP-address changes each 24 hours, or when I reconnect to my provider. But try to explain two or more Smart TV's which are actually stationary with each having a different IP-address streaming Netflix at the same time. All that stuff is being logged. You can also verify this on your own for the last 60 days.
It's irrelevant what is being or not logged. You mentioned the same IP address being in use and that's not at all the case.
Besides if I use a new tv, ps4, phone, etc every single day with my account, Netflix is doing nothing about it. That's why they sell the plans with limit of screens using the account at the same time.
Meaning, I don't need to explain anything to Netflix on my usage lol