Many ISPs use traffic shaping so they can artificially throttle streaming sites and Netflix works by adapting to variable network conditions by lowering (and raising) the stream's bitrate. The ISP's aim is to lower traffic and the result the user can watch more content albeit at lower bitrates. This is part of the net neutrality debate.
Yah, my isp doesn't throttle netflix. But if they did, there's be even less of a reason to buy a 4k tv or rely on streaming for UHD content.