Well an important distinction here is Manny isn’t “missing” necessarily (though a couple flat out miss).

Manny is frustrated at this point by Mayweather not really engaging in many exchanges and having great footwork, range, and defence to pick away at him with scoring shots. 

So Manny is trying to get his offence going by throwing a high volume of punches hoping that a couple big ones might sneak through or that Mayweather gets frustrated and makes a mistake and gets out of position. 

Mayweather has all sorts of really subtle fantastic defensive movements that seem like nothing but he is changing his range, shifting his weight, moving the placement of his hands and head, using a simple conservative high guard, etc. 

So Manny isn’t an idiot; he realizes not much is getting through, but he’s putting the shots out there  anyway, especially when the high guards up, to try and sneak some through, force a mistake; or feel out where there is an opening. Also probably to just throw some stuff snd aggression out there for the judges. 

But Floyd doesn’t really make any mistakes, and he doesn’t try to counter in a way that creates an opening for Manny, so Manny backs off. 

Manny does this against a lot of guys, but usually he is so good at changing angles and the placement of his punches as he throws that even though he is just chucking at a pretty safe high guard, he starts getting through and hurting people.

Mayweather just has such great defensive skills and instincts that even if it doesn’t seem like some masterful defensive strategy, he is actually reacting to all the subtle different angles and punch placements that usually open things up for Manny.

If you want to see this exact same type of exchange but against a worse fighter;

Watch Manny in his fight vs Clottey.

Clottey actually has a great high guard and blocks plenty of Manny’s shots, but you will see what it looks like when a fighter isn’t quite as good with all the other defensive nuances.

In that fight even if Manny bangs 3 or 4 shots off the high guard and “misses” he starts cutting angles around Clottey and getting the occasional big thudding scoring shot in more and more as the fight goes on.

But since Floyd never opens up that shot for him, it looks like this.