I think that (and this is my own, completely unproven and unscientific opinion) -
- some men's "girls swimmers" are better swimmers than their "boy swimmers" (or vice versa) ("boy swimmers" are the sprinters, "girl swimmers" the endurance) so it could be that some are better at their jobs than others
- female environment definitely plays a part (and will favour some sperm over others)
- add in external influences, like diet, medication and temperature and you can affect sperm quality and cervical environment
- and of course timing
I also think that for us, either DH didn't produce good XY sperm (boy) or either I can't carry them, because we tried EVERTHING to sway for a boy (and I know that it isn't scientifically proven even if it works, but it "seems" to work when you follow it closely) and whilst I did fall pg, it was a m/c, and I am convinced that my m/c have been boys (no evidence, just a feeling). We gave up the "swaying" and just went back to our "usual" (which according to gender swaying naturally swayed towards girls anyway) and ... well, we have 2 DD and 2 more DD on the way. team pink here!

BTW - I work with several people who have in their first marriage had 2 x girls (or boys) only to have in their second marriage 2 x boys (or girls).