IMO.

Both cubits existed. The distance from Behdet to syene of 106 It'r. The length of egypt being 100 It'r of 15,000 royal cubit of 28 fingers[7 degrees]
with egypt cut into 6 + 1 degrees for upper and lower egypt.
The 6 and 7 palm cubits therefore relate directly to unification.
This 6 palm cubit however uses longer fingers and bigger palms in the ratio of 24:25.
This 6 palm cubit thence becomes the equivalent of 25 original fingers [a la Herodotus].

These same fingers form the basis of the typical remen diagonal of
20 x sqrt 2 fingers along with the likes of the "black cubit" of stechini at
7 herodotus foot = 4 "black cubit" of 21 inch (21/12 herodotus foot)
6 herodotus foot = 4 common cubit
1 fathom being 4 cubit (common or royal) or 6 feet or 7 feet.

12.154" (308.703mm) Herodotus foot (25/24th of an Attic/Roman foot) [12"]
18.230" (463.054mm) Herodotus common cubit [18"]
21.269" (540.229mm) Stechinis' Egyptian "Black cubit" [21"]
14.584" (370.443mm) Egyptian Remen [14.4"]
20.625" (523.886mm) Egyptian Remen royal cubit [14.4 x sqrt 2 inch]

best,

Mick