If he, as he stated, had her over his shoulder, carrying her, then placed her on the ground face up, it's possible the back of her t-shirt could have some of his hair. The rain didn't necessarily run in rivets under her body. The soil became soaked under her body, but the ground under her body did not necessarily run like a river, washing away every last dna instance. I remember Iowa soil to be very porous and soaks in rain.
My question is more to his thinking on the matter. Obv. we won't know his thoughts. I am trying to consider the crime visually, and psychologically, questioning what, if any, precautions he took in terms of evidence. How far ahead he thought of doing the crime, when & how concealing the crime occurred to him, & if that concealing was more in the moment or in advance.