usu defined as "the cardinal number that is the sum of five and one"

(1) Why is it so defined; why not "the sum of three and three," or "the whole number between five and seven," etc, or even just "six"

(2) Why don't I find it in Random Unabridged

Incidentally I acquired this huge dictionary hoping to find all the obscure words that aren't in Merriam Collegiate, but I have been sorely disappointed in this respect and in other ways too. I wonder if anyone else has has had the same reaction as I

Funny, the two dictionary entries I looked at said it is the number six on a die, or the throwing of a six with a pair of dice.

The definition of "sum of five and one" is from Webster's. The earlier versions are out of copyright so other dictionaries have quite likely plagiarised it.

I thought it was only related to six on a die or dice, too.