TTF vs OTF rendering difference on Windows (darkness/boldness). Why?

I've noticed that web fonts render differently in browsers on Windows (7 and 10, using DirectWrite IMHO) depending if there's a TTF or an OTF converted to WOFF. Can anybody explain why this is happening?

You can test the difference here:

https://rawgit.com/herrernst/31b75f42bc12acc7c8be70de01ea237a/raw/4d3456711b90c107d9d52619b0bb0305440d905d/index.html

Or on this screenshot: