Different window sizes?
I believe those brackets are from where they removed the pane of glass from the cladding panels. They look like they're in place to keep the pane from
IRT Lexington Avenue Line at Astor's Place in Lower Manhattan
IRT Lexington Avenue Line at Astor's Place in Lower
World Trade Center Transportation Hub , WTC 1 & 7 in Lower Manhattan by Nexis4Jersey09, on Flickr
"It's not often that a police officer will explain to you why what you're doing is perfectly legal just before he puts you in handcuffs"