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.
Just want to point out that MAC address information is not shared over an HTTPS connection. It's possible if you download their Netflix app that they can get access to this data, but even so, that's sort of irrelevant, as that doesn't give away any information about where the device is located. Even moreso than that, Netflix is a company, they aren't a government, so they don't have ISP records to look through or anything more than you or I do.
But if you think about it logically, why would a company like Netflix go through all that trouble when the power they *do* have allows them to just ban these accounts. I would bet that they have automated alerts for when accounts get unusual activity, and the obvious solution to me would be to just ban those accounts before it got out of hand. You think they would try and identify a handful of individuals (who may not have much money anyways given their use of a stolen Netflix account) and take them to court for a negligible amount of money.