I have done a little research prior & came across that the sperm for males swim quicker than the sperm for females. So, depending on how close you are to ovulation, I think that's how it comes about. Also, some places do gender selection. They spin it extra during sperm washing for more/mainly the male swimmers. Hope that helps.
Thanks for posting this. I don't know the answer, but was trying to find info on this since I just had my first IUI last week!
My son was conceived from IUI... I'm also now 11wks pg from IUI again... check back in about 7wks and I'll let you know...
I don't know of any statistics on this, but it would be interesting to know if you found any. Have you tried googling it?