The fertile (spore bearing) fronds are upright through the summer season - the fronds without spores on them are produced first by the plant in the spring and then later in the season Christmas Ferns produce spore bearing fronds. The non fertile fronds are the ones that fall and lay on the ground. I can take an educated guess that the reason this fern has upright fertile fronds is so it can take advantage of the air movement to disperse its spores. Usually the weight of the snow in winter will cause the fertile fronds to be more horizontal than vertical by their second season.

Having both upright and prostrate fronds on this fern is a normal characteristic of a healthy plant.