this looks to be it is the strain, as it looks you are not doing anything wrong so stop trying to figure it out,
sometimes with each seed we pop we sometimes will
run into one that does not look right, sometimes we get mutants, runts, and just slower growers.
it is hard to say just why, theories could be had trouble getting out of her shell when she popped, the seed could have come from a lower branch perhaps the seed was underdeveloped
or maybe the seed did not cure right, so much. so chalk it up to a learning experience and remember no two plants are ever the exact same.
they are living plants we cultivate for those nice juicy buds.
so stop stressing, let the plant grow in to what she will become.
just remember not to over water and i am sure this one will produce some good bud for you, and you now have a special connection to this one.
she will be your baby. enjoy the grow