Clive,

Unfortunately a few decided to take your inquery beyond scope, here I hope to bring it back as no real answer has been offered to you.

I do not consider myself an expert in the metrological sense but I do have my own ideas, taken in this context please go easy on me... (grins)

It is perhaps the most original phenomena of how the egyptians handled measurement, I am speaking of unit fractions (i.e. reciprocals), they must have realized the infinite reach of division for any one thing, perhaps the nexus of Thoth's creation of Heh. But if we apply this concept to dividing the royal cubit into 28 parts we are left with your big question "why?"

With only the ability to divide by 2, 4 and 7 evenly or into equal parts it leaves us with parts that look like this...

1/7 ~ 1/4 ~ 1/2

Now we know of the old.... 1/6 + 1/3 + 1/2 = 1; thus (1/12 + 1/3) + (1/12 + 1/2) = 1

if we take the parts from 28 we get.... 1/7 + 1/4 + 1/2 = .892857143

And.... .892857143 x 28 = 25

dividing this into two parts by dividing the odd denomintator (1/7) we find...

....... (1/14 + 1/4) + (1/14 + 1/2) x 28 = 25

....... 28(1/14 + 1/4) = 9

....... 28(1/14 + 1/2) = 16

....... 28(1/28 + 1/4) = 8

....... 28(1/28 + 1/2) = 15

Just a thought for your imagination to ponder I don't profess it as an answer but a different lens to look through. Though division is predicated in a modern sense as being into equal parts, in a system of unit fractions this is not the case IMHO, division is division on all fronts so long as the combined component sum equals the desired sum. This is why 3 + 1/8 + 1/60 has a chance.

Best Regards,

B.A. Hokom