BECAUSE ALL IN ALL IS SOME. When you are being subject,you knows all about subject. But when you are appearing and being ALL AMONG ALL other/s, then you will prevail as SOME(anything of anything) which is realisation that you don't(will never) know anything completely about subject. That is why one has to review about "WHO AM I ?" again and again while being human by becoming human again.

WHEN YOU KNOW SUBJECT COMPLETELY,THEN ONLY YOU WILL REALISE THAT YOU DON'T "ANYTHING OF ANYTHING" ABOUT SUBJECT.