Judging by the data usage it clearly isn't actually streaming video as you say, perhaps it could be traffic flows that are passing through the switch (DNS and TCP SYN's etc) which the Meraki switch recognises as Netflix, but are then blocked as they hit your firewall.
The flows are still present with clients trying to reach Netflix, but are simply passing through the switch then getting terminated.
Just my 2 cents on how I see it, someone may be able to confirm for definite.