I don't know how your program works but if it is something that kids/parents choose to attend I have a possible theory based on my experience with my DS and DD.
DS clearly doesn't fit well in an average classroom and when he doesn't have a good fit he doesn't hide it. We have had many, many discussions with teachers about his behavior over the years. I would move mountains to find a better fit for him and have him ride a bus across town, rearrange my work schedule and give up him going to school with the neighborhood kids (which is something that is pretty important to us).
Now I don't have scores for DD but we've had more comments about her being smart/gifted than we've ever had about DS. School is way too easy but she handles it in a completely different way. Knowing what I know about gifted education I would probably move her if I could but I don't have the same urgency that I have with DS. I could easily see many parents in the same boat not choosing to move their kid to a program if they are compliant teacher pleasers such as my DD.
Now I realize that not all boys are like my DS and not all girls are like my DD but I know quite a few others that would follow the same general pattern. Perhaps there are more girls that qualify but simply don't attend?
