Daylily crowns are very compressed stems. When a daylily fan grows the crown becomes taller like a normal stem. If the crown does not change position in the soil (if it does not move up) then either the old lower parts of the crown must disappear (rot away or be absorbed, etc) and the crown settles, or the crown pulls itself lower or perhaps some of both.
I went out and dug up a few fans from a clump of Wynnson (has not been disturbed for 15 years) and the photo below shows one fan's recent growth.
There are probably four year's of crown growth visible in the photo and I don't know how much broke off at the bottom. There are two year's of roots.
Wynnson has not moved up in the soil during those 15 years.
