This is nothing new. Fleet Admiral Shanthi in Redemption is actually specified in the script to be the head of Starfleet, yet she only wears the three-boxed pips of a Vice Admiral. There's also Fleet Admiral Brackett in Unification who also only wears the insignia of a Vice Admiral.Nechayev was a vice admiral in her first appearance and wore the rank of that,
Yet when she was promoted to full admiral she wore the rank of vice admiral, as seen in a couple of episodes of TNG
In 2371 she became a fleet admiral and when she appeared in the ds9 episode the search part 2 as fleet admiral she wore the pips of vice admiral....2 ranks below.
In fact every appearance of hers was her wearing the vice admiral despite 2 promotions in between her appearances.
IMO, Admiral Leyton should have been a four-star Admiral at least, but he too was only a Vice Admiral.
The only time in all Star Trek we actually do see an actual five-star Fleet Admiral is Admiral Marcus in STID.