Board Thread:S,R,&G/@comment-153.150.21.222-20170301065626/@comment-77.234.44.170-20170301171444

Actually, it's explained by Reki Kawahara in his afterword found in the first volume Progressive that is in line with the canon of the main books, even though he had to soft-retcon a few things. As for Sachi,  Reki stated on Twitter that she is no longer his first love like in the WN.