This is only humorous speculation and I don't really believe it...I hope that wards off any bashing.
It is said that this is the highest concentration of midiclorian (may not have spelled it right) ever found. This could mean a couple of different things:
1. the force was used to block out Shmi's memory or make her unconscious. This way she may not think she did have sex and lead to the virgin birth idea.
2. It was a very powerful Jedi that nocked her up. Who is the most powerful Jedi that we know....YODA. Think it was someone else did you mmmm.