FNA was forked from MonoGame by flibitijibibo. While MonoGame uses OpenTK for desktop platforms and supports additional platforms with platform-specific APIs, FNA has been streamlined (support for all mobile / console platforms has been removed) and it simply uses SDL2.
OpenTK was until recently an unmaintained C# project. SDL2 on the other hand is a very robust C library, it's been used to ship many commercial games on Steam on all desktop platforms. On top of that, flibitijibibo is being paid to port many XNA games so he's had time to inspect, clean up and improve almost all aspects of the framework in a more methodical fashion that MonoGame's organic development style.
How is the accuracy of porting is achieved? Is this the clean room kind of development?