One twin delayed?

Tiffany • Mommy of 5!
Has anyone had one twin significantly behind the other?
My twins are 10.5 months, and we always thought my boy would be a little behind because he was in the beginning. But it's actually my girl that is, and they say my boy is on target and a little advanced with his adjusted age. 
We had early intervention evaluations for my girl twin and she has signs of hearing delays, and 25% delays in 3 other areas. 
She just started pulling up on furniture just last week while my boy had been doing it for 2 months. She started crawling last month, he's been doing it for about 4 months. 
Anyone else have experience with one more delayed than the other?