I think it's because of the bookcase in the background.
Could be that with a lower chair the thick horizontal line formed by the bottom part of the bookcase would cut through his head in the background. Bad TV optics.
If so, the bookcase must have to be included if they are adjusting his seating to get the bookcase in.
Putting on my conspiracy hat, could he be messaging through the books/pictures in the background? I'll guess we'll have to look back through older videos to see if anything is changing.
He is getting intel briefings as candidate, correct? Can he legally pass on what he is briefed about to the dem suits?
I'm not sure how intel briefings fly if Biden is a suspect in a criminal probe (Ukraine/Spy Gate).