1. This is normal. The data may not totally reflect our intuition. The task of attribute
selection based on "your understanding" v.s. that based on IG is for you to get something
like this, so that you would hopefully have a deeper impression of knowledge learnt here.
2. This is the "hyper-parameter selection" problem, similar to selecting min_support. It is
up to you. You can choose whatever value you are happy with.