Main Universe Batman had actually noticed it earlier: although the Batmen were broadly the same, there were still differences in the details—different ages, different temperaments, and many of them did not look quite alike.
Main Universe Batman sorted all the Batmen's appearances into several categories. The first was those who looked almost exactly like him. The second was those who should have looked exactly like him but differed slightly because of age. The third was those who should also have looked like him, but whose vastly different temperaments made them seem unlike him. The fourth was those who did not resemble him at all.
That bastard was a combination of the second and third categories. In theory, they should have looked exactly alike, but their different ages and temperaments made them genuinely seem rather different.
Main Universe Batman had also noticed a problem with Bruce's age. Over the past few years, he did not seem to have aged at all. He suspected Bruce might have acquired some mysterious power that had frozen his appearance in his twenties.
Of course, it was also possible that no matter how old the guy got, he still looked like an uneducated layabout, making him seem young.
At that moment, another Batman who was happily chatting with Mr. and Mrs. Wayne belonged to the third category. He actually looked exactly like Main Universe Batman, but their temperaments were so different that they did not seem alike.
The greatest difference was that this guy was a little too sunny. Not in Bruce's unhinged way, but in an utterly normal and healthy way.
To what extent? When Main Universe Batman stood beside him, he always felt like he himself was the one in the wrong.
The last person who had made him feel that way was Superman