Why do I not get 100% Accuracy with KNN with $K=1$
In your training set (X_2,y), there are some samples with the same input features X_2 but different labels y. For example, the 73rd and 147th samples, which are labelled into class 0 and 1, respectively, have the same input values [6.3, 2.5]. There are more samples like this in the dataset. Therefore, you could not construct a perfect classifier for such data.