Ever since Viewer 2 was released in 2010, Second Life's avatar height has never been accurate or equal to prim height.
Prim height had been the standard used in earlier Viewer 1 viewers before 2010.
Fifteen years later, we're now using Viewer 7, which still retains the false avatar height measurements from Viewer 2. Our modern avatar shapes are still taller than a comparable shape created from before 2010 in v1 viewers, or than a modern prim of same said height.
Try it for yourself:
  1. Download any of the v1-based SL viewers (such as Singularity Viewer or Cool VL Viewer) and experiment with making an avatar shape with a height equal to that of a 5'10" (177 cm) tall prim (Note: this should be done without wearing a shoebase)
  2. Change to a Viewer 7 based viewer and see how the described height in a v7 viewer using this same shape has increased, yet the prim retains it's correct height!
I propose that described Avatar Height in all current viewers (via Appearances menu) should be re-syncronised to the previous prim height as per pre-2010 v1 viewers.
Not only would this be more accurate, it would also be beneficial to those creators who build houses or other structures where ceiling and doorway heights matter.