"All" and "No" statements are very strict and hard to prove true. On the other hand they are relatively easy to prove false. Since "Some" statements are very loose and flexible the opposite holds. "Some" statements are easy to prove true and hard to prove false. The same holds with "Some....are not...." "Some birds do not fly" can be proven true with a single example. Find one penguin or ostrich and you have proved the statement to be true. However, consider how hard it would be to disprove the statement "Some American high school English teachers have never heard of Shakespeare." Even finding thousands of American high school English teachers who have heard of Shakespeare wouldn't prove the statement false. If there is just one out there who hasn't heard of Shakespeare, then the statement would be true. I can't definitely say it is false until I check out every single English teacher. And that would be very hard to do.
So "Some...are not...." statements are easy to prove true. One example of something that is a member of the subject category but not a member of the predicate categories proves the statement true. However, so long as I have not examined every single member of the subject category, no matter how many negative instances I find, I will not have proved the statement false.
But as with "Some...are.." statements avoid the common mistake of thinking that single negative examples prove them true. Some people assume that "Some cats are black" and "Some cats are not black" either mean the same thing or imply each other. This is not right. Finding a black cat doesn't prove that some cats are not black.