Maybe he can't see them but can only see the partial deflection of the bolts.

There is a big thing made in the PT about the power of the dark side focused in a few individuals blinding the many Jedi to the powers of the Sith.

Maybe this works in reverse.

Only one Jedi and two Sith means the balance has shifted and Palpatine's cock-sure powers are weakening.

So when he refers to Luke's feeble skills he is assuming Luke is trying to reflect the blasts himself and not that he is getting some limited help.

Vader on the other hand may be able to see them as he holds the scales.

I was suggesting making it subtle (Monroville found my mockup, not the last word hopefully in the concept, too subtle to make out in a frozen image).

Palpatine is so snarly that he could notice and still keep the same expression and say the same things.