Also, not enough is made of the fact that the numeral system was based upon United States decimal coinage. Viewed in terms of 1 cent, 5 cent, and 10 cent coins, with the added notion of a barred gate for 1 to 5 (as explained), the numerals actually make a lot of sense; especially if one considers them as erosions into arcs and squiggles caused by rapidly drawing the original full circles and bars.
* 1 to 5 are the five-barred gate with the downward bars mostly elided or reduced to squiggles for speed and the pen not removed from the paper in between bars.
* 6 to 10 are a 5 cent piece reduced to an arc plus one to five 1 cent pieces as bars, again mostly elided.
* 11 to 15 are a 10 cent piece still mostly a circle plus one to five 1 cent pieces.
* The 30 to 90 symbols are the superscript notation described in Lowery's biography, but prefixed instead of suffixed. 50, for example, is a prefixed 5, as reduced to a squiggle, before a 10 cent piece.
The text states that numerals above 20 had the numerals from 1 to 9 appended. By implication, therefore: 16 to 25 are a 10 cent piece, plus a 5 cent piece, plus one or two barred gates of 1 cent pieces; and 26 to 29 are a 10 cent piece, plus two 5 cent pieces, plus some 1 cent pieces.
It would appear that Sequoyah didn't encounter 25 cent pieces often enough to warrant retaining them in the system as it evolved.