Why this number specifically? I don't know...
But why is it important to take a subset of the features and not the whole set of features? It's because if you always take the same features (say the whole set of features) you will get highly correlated decision trees in every iteration, and thereby will not be able to cancel out their inherint great varience.
I beleive that the features are sampled in a uniform fashion, which means that if you have many trees, approximately all features should be represented equally over all of the trees.
However, in your case the subset of the features has the same size of the original feature set ( ceil(sqrt(2)) = 2 ). Once the set of features is selected, a certain criterion is used to select which feature should the split be based on. The criteria can be the Gini index, or information gain (entropy).
So my guess is that since you're always ending up with the whole set of features, and everytime the same criterion is used to choose which feature to go with, you're always ending up with the same feature, and the other one is excluded.