The docs for Identity Purification say the following:

If the argument is 0, return Null. If it's Null, return 0, Otherwise, return the argument

But the sequence 1 Identity returns Null because of the second half of this line. This seems like a mistake, and it makes Identity much less useful (ie. you can't use it to coerce 0 to null in a comparison, because it'll be null either way).