DIV=n method consists in considering the available paper area (binding correction removed) formed by n2 equal smaller rectangles, by dividing both width and height by n. The upper horizontal stripe and the two lower ones make the upper and lower margin. Similarly the inner vertical stripe and the two outer ones make the inner and outer margins. However, when the
oneside option is active, one fourth of the outer margin is transferred to the inner margin (so both are one and a half stripe wide). The upper and lower margins still follow the rule.
DIV=9 on A4 paper, we get a text width of 6 times 21/9 centimeters, that is 14 centimeters, as you found out.
\oddsidemargin turns out to be 27.31467pt that, added to 1 inch, give exactly 3.5 centimeters. The computations are a bit involved because they must take into account the one inch shift in both vertical and horizontal directions that DVI drivers (and pdfTeX too, for compatibility) always do when printing or previewing a document.
twoside option is active, instead,
\oddsidemargin is –5.88022pt; adding 1 inch gives 2.33333 cm; the outer margin the double (4.66667 cm), making again for 14 cm textwidth.