curled apple tree leaves?

We planted some apple trees this january when we got them. The trees have came out fairly well and even had a few blooms on them, but I'm a little concerned that some of the leaves are curled a bit. They are curled from side to side and not twisted. They are not curled up all the way, but it is noticeable.

The soil is a clay majority, but I mixed leaf mulch in the hole with the soil before it was planted and then top dressed with leaf mulch.

Normal or lacking of something?

thanks!