Sherlock is such a bad liar. He says he researched John before he met him, yet he had no way of knowing that Stamford would meet John that day. Also, he got it wrong - if he’d researched he would have known Harry was a sister and being Sherlock he would have wanted to show off, so he wouldn’t have purposefully made that mistake.
Dear Sherlock,
I can see clearly now, the tears have gone.
No wonder John doesn’t believe him. ;)
I still love you!