In this case, I am sure it's not because he's a brain-addled old man, but rather that he is deliberately lying and hoping the dupes who somehow still support him will go along with it. 

Here's today

"Not that it matters"? Really? Oh, I guess if you lie every day about something and continue to do it, it doesn't matter to YOU, but it does to honest people.And Trump was RECORDED saying that's why he did it.