I think that as the shortlist is decided by adults,it is often the case that, although they appreciate the value of the books, that the students do not enjoy them as much as other books they read.
Many of the shortlisted books over the last few years have male characters as their 'leads' and are what i would consider 'boy books'.
This year who can deny that 'Here lies Arthur' could be a boys book, its about medevil war! also Crusade is clearly aimed more at boys than girls. Gatty's tale seams very girlish, but imo it is not a good book anyway. 'Ruby Red' applies equally to both genders, despite its main character being female. What I Was has a male lead also. ditto finding violet park.
Maybe boys just find it easier to say 'it's not ym type of book' then to try and explain what they find wrong with it.
If the children choose the shortlist they would choose books they enjoy readin such as Robert Muchamore and Jaqualine Wilson. I do not think gender target groups have any impact on the greatness of a book.