Earlier this week, Microsoft released a blog post claiming that its own browser, Microsoft Edge, was the only option for streaming Netflix in 1080p on your PC. Websites set out to fact-check this claim for themselves, and found it true.

According to PC World, Chrome, Firefox, and Opera all cap at 720p in the real world while only Edge can stream up to 1080p. Netflix won’t stream 4K to any PC, even if you have a gigabit fiber connection and a 10-core CPU + GTX 1080 to decode it with. If you want the highest video quality in Microsoft Windows, you need to either use Edge, Internet Explorer 11, or the Netflix app itself to get it.

Why is this the case?

The short answer is that nobody seems to know. The slightly longer answer is that Netflix seems to be somewhat unique in this category. Amazon Prime doesn’t give any indication that it only streams in 1080p to specific browsers.

In the absence of official statements on the topic, viewers have theorized that it’s the result of specific DRM hooks in Netflix that only allow for 1080p streaming in Edge. Microsoft certainly promotes this view — the company’s blog notes that:

From video hardware acceleration to PlayReady Content Protection and the Protected Media Path, Windows 10 is designed to provide the highest quality, most secure, and most power-efficient video playback available on any version of Windows.

This, however, seems to either be an elaborate smokescreen or far less than the full explanation. The WayBack Machine offers a window to the same page from February 2015, before Windows 10 had ever launched. At the time, Windows 8.1+ and Internet Explorer 11 were listed as providing up to 1080p, as was Safari, while Chrome was still stuck at 720p. (Firefox isn’t even mentioned).

This doesn’t mean that there aren’t still browser-level DRM protections that make the difference between supporting 1080p on IE11 and not supporting it in Chrome or Firefox — but it doesn’t appear to have anything to do with new protections baked into Edge or Windows 10. Other arguments we’ve seen online have centered around NPAPI or Silverlight support, but neither of these holds up under scrutiny.

If you’ve got insight into why Netflix won’t or can’t stream above 720p while the comparatively ancient IE11 has no trouble doing so, let us know. The fact that other streaming services seem to get around issues caused by DRM implementations has clouded the issue, and it’s not even clear if this is a problem related to continued support for older operating systems or for plug-ins and features that would require updates to the browsers themselves. Unlike Microsoft, Chrome and Firefox continue to support Windows XP — and it’s possible the DRM features in question aren’t baked into that operating system, leaving these options less secure for content streaming.